The Massasauga, Sistrurus catenatus, is a species of venomous rattlesnake found in the United States, primarily in the Great Lakes region from New York to Illinois, including Ontario in Canada. Some isolated populations are found as far south as northern Missouri. It is the only venomous snake in Michigan, where it is known as the Michigan Rattler. It is also called the swamp rattler because it prefers swamps and other wetland areas. Its common name is derived from the Native American Ojibwa name of the snake, meaning “great river-mouth.”

The Massasauga is not a large snake, with adults ranging from 18 to 30 inches in length. Its color pattern consists of a gray or tan ground color with a row of large rounded brown-black blotches or spots down the center of the back and three smaller rows of alternating spots down each side. There are also cases where the back blotches join with those on the sides. The young are well-patterned but paler than the adults. This is also the only Ontario snake with vertical pupils. There are also heat-sensing pits on each side of its head.

This is a rather shy species of rattlesnake and avoids humans when it can. Most massasauga snakebites in Ontario have occurred only after people deliberately handled or accidentally stepped on one of these animals. The venom of rattlesnakes contains special enzymes that disrupt blood flow and prevent blood clotting. Severe internal bleeding causes the death of the small animals that this snake eats.

In Ontario the massasauga rattlesnake is found only near the eastern shore of Georgian Bay, the Bruce Peninsula, Wainfleet Bog and Ojibway Prairie. Although it also ranges through several American states this is its only range in Canada. The rattlesnake is becoming rare in Canada due to persecution and loss of habitat, and is designated as Threatened by the Committee on the Status of Endangered Wildlife in Canada COSEWIC and the Committee on the Status of Species-at-Risk in Ontario (COSSARO).