Colorado has 3 venomous snakes named by official sources: Massasauga Rattlesnake, Midget Faded Rattlesnake and Prairie Rattlesnake, as labelled by Colorado Parks and Wildlife. It also lists 11 snakes as mildly venomous or not dangerous to people. This guide covers 9 animal groups from Colorado Parks and Wildlife (CPW) lists and other official sources, with photos, venomous status and when each group is active.
Identification help has limits: a photo cannot confirm that an animal is safe. Watch from a distance and never handle a wild animal.
These are the animals official sources describe as venomous or as a risk to people in Colorado, across 5 groups. The reason is given in the source's own terms.
Painful stings when nests are disturbed (CSU): “If their nests are directly disturbed, harvester ants will vigorously defend the mound and are capable of very painful stings.”
Disease: “Culex tarsalis, one of the main West Nile virus vectors, commonly occurs up to altitudes of 8,500 feet and is found as high as 10,000 feet above sea level.”
Aggressive if approached, especially cows with calves and bulls in rut (CPW): “Cows can be dangerous if approached or caught off guard, so keep dogs on leash and avoid forage areas like thick willows when recreating.”
Disease: “Tularemia is a fatal bacterial disease that infects rodents (particularly beavers), rabbits and hares; it can also infect humans and domestic animals.”
Official sources say the brown recluse is not native to Colorado or not found there. It is not counted among the dangerous animals above; each entry quotes the source.
Brown recluse: not native. “Funnel web spiders are harmless but are often mistaken for the brown recluse, a spider whose venom is of medical importance, that does not naturally occur in Colorado.” Colorado State University Extension
Darker cells are the months with more open-licence observation records of each group in Colorado (GBIF). They show when people see and record these animals, not how many there are. The current month, this month, is highlighted.
Spiders often wander indoors in early fall; funnel web spiders are most common in homes in late summer and early fall. Colorado State University Extension
Report: Invasive species: file a report with CPW (online "Report a Sighting" form linked from cpw.state.co.us/invasive-wildlife). Injured/abandoned animals, feeding or illegal activity: CPW Denver Headquarters 303-297-1192 or any CPW office; anonymous: Operation Game Thief 877-265-6648. After-hours emergency: Colorado State Patrol 303-239-4500. Wildlife posing immediate danger: local CPW office or law enforcement. Colorado Parks and Wildlife
Poison Help: 1-800-222-1222 (Rocky Mountain Poison & Drug Safety toll-free number listed by CSU Extension; national Poison Help line per HRSA). Colorado State University Extension
If you hear a rattlesnake, freeze until you locate it, then get at least five feet away; never try to catch, kill, provoke or move it. Colorado Parks and Wildlife
Rattlesnake country: wear sturdy footwear and long pants; watch where you put hands and feet. Colorado Parks and Wildlife
Mountain lion: if the lion does not leave, face it and slowly back away; fight back if attacked. Colorado Parks and Wildlife
Questions people ask about Colorado wildlife
Are there venomous snakes in Colorado?
Yes. CPW says there are three kinds of rattlesnakes in Colorado (prairie rattlesnake, midget faded/western rattlesnake, massasauga); CPW's Quick Key lists these three as the venomous snakes of Colorado. Colorado Parks and Wildlife
Are there copperheads or cottonmouths in Colorado?
Not on the official list. Neither is in CPW's list of Colorado snakes; CPW names only rattlesnakes as venomous. USGS NAS has one non-native Cottonmouth record (collected 1965, Adams County). USGS Nonindigenous Aquatic Species (NAS) database
Rarely. CSU Extension says brown recluse and all other recluse spiders are extremely rare in Colorado; sightings often come from spiders carried in from other states. Colorado State University Extension · More
No. CPW says grizzlies were extirpated in Colorado in the mid 1950s and no longer live in Colorado. Colorado Parks and Wildlife
Are there wolves in Colorado?
Yes. CPW says gray wolves have an established population in Colorado for the first time since being extirpated in the early 1900s. Colorado Parks and Wildlife
CSU Extension says Lyme disease is not present in wild animal populations in Colorado; Colorado tick fever is by far the most common tick-borne disease. Colorado State University Extension
Are there kissing bugs in Colorado?
Yes. CSU Extension has a fact sheet on conenose (kissing) bugs in Colorado; they feed on the blood of wild animals, especially rodents. Colorado State University Extension
Sources
Species Profile List. Colorado Parks and Wildlife. page states 2026-04-09T13:47:52-0600; checked November 3, 2026.
“This is a rear-fanged snake that is venomous, but it is considered non-dangerous to humans.”
Scorpions of Colorado. Colorado State University, Department of Agricultural Biology (Extension entomology). undated page; checked November 3, 2026.Quoted passages (2)
“Three species of scorpions are confirmed to occur in Colorado.”
“Birth of the northern scorpion usually occurs around August.”
“Bites from the widow spider are painful and potentially dangerous because they contain a nerve venom.”
“Their bite can be painful but they are not aggressive and their venom is not known to cause medical problems.”
“Funnel web spiders are harmless but are often mistaken for the brown recluse, a spider whose venom is of medical importance, that does not naturally occur in Colorado.”
“Widow spiders, particularly the western widow (Latrodectus hesperus), are common in Colorado.”
“Tarantulas are found in southeastern and southwestern Colorado.”
“Funnel web spiders are the most common spiders found in homes, particularly during late summer and early fall.”
“They occasionally enter buildings, particularly in early summer.”
Tarantula Hawks of Colorado. Colorado State University Extension. Published August 2025; checked November 3, 2026.Quoted passages (1)
“They have among the largest quantity of venom per body mass of any insect (Schmidt 2004).”
Velvet Ants (Colorado Insects of Interest). CSU Extension, Western Colorado Insects (Tri River Area) - Colorado Insects of Interest. undated page; checked November 3, 2026.Quoted passages (1)
“…the implied claim about its ability to kill a cow is absurd and their venom is not particularly toxic.”
European Paper Wasp. Colorado State University Extension. Published June 2013; checked November 3, 2026.Quoted passages (1)
“Several yellowjacket species are native to Colorado and these historically have been the most significant stinging insects of the region.”
Harvester Ants (Colorado Insects of Interest). CSU Extension, Western Colorado Insects (Tri River Area) - Colorado Insects of Interest. undated page; checked November 3, 2026.Quoted passages (1)
“If their nests are directly disturbed, harvester ants will vigorously defend the mound and are capable of very painful stings.”
“Culex tarsalis, one of the main West Nile virus vectors, commonly occurs up to altitudes of 8,500 feet and is found as high as 10,000 feet above sea level.”
“By far, the most common disease that is spread by ticks in Colorado is Colorado tick fever.”
“Fortunately, the most important tick-borne disease in North America, Lyme disease, is not present in wild animal populations in Colorado (i.e., not endemic).”
Living with Bears. Colorado Parks and Wildlife. page states 2026-10-06T10:46:47-0600; checked November 3, 2026.Quoted passages (2)
“Black bears seldom ‘attack’ or behave truly aggressively, but will fight back if they feel threatened.”
“If you’re attacked, don’t play dead.”
Living with Mountain Lions. Colorado Parks and Wildlife. page states 2026-01-21T13:33:35-0700; checked November 3, 2026.Quoted passages (2)
“Lion attacks on people are rare, with fewer than a dozen deaths in North America in more than 100 years.”
“If the lion does not leave the area, face the lion and slowly back away. Fight back if attacked.”
Moose. Colorado Parks and Wildlife. undated page; checked November 3, 2026.Quoted passages (2)
“Cows can be dangerous if approached or caught off guard, so keep dogs on leash and avoid forage areas like thick willows when recreating.”
“Dangerous conflicts between people and moose are increasing.”
Disease Exposure. Colorado Parks and Wildlife. page states 2026-06-26T09:19:37-0600; checked November 3, 2026.Quoted passages (3)
“Rabies is rare in all mammals in Colorado, but bats and skunks are the most commonly infected.”
“Plague is a bacterial disease that most commonly affects prairie dogs.”
“Tularemia is a fatal bacterial disease that infects rodents (particularly beavers), rabbits and hares; it can also infect humans and domestic animals.”
“Females give birth to their young beginning in late August or early September.”
Living with Wildlife in Colorado. Colorado Parks and Wildlife. page states 2026-05-01T10:57:56-0600; checked November 3, 2026.Quoted passages (2)
“There is an increase during the winter migration season, especially between dusk and dawn hours, when animals move to lower elevations for the coming winter.”
“You may see more young wildlife during spring months in Colorado, including baby deer, elk, pronghorn, moose, rabbits, foxes and birds.”
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