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Studies of natural history are important for determining baseline information, particularly for species that might be threatened or endangered. We collected 254 (141M:94F:19UNK) massasauga, Sistrurus calenatus edwardsii, from May 1995 through October 1996. The average snoutvent length (SVL +/- 24 mm). Habitat descriptions indicate that the massasauga in Colorado is a semi-arid grassland species that utilizes areas of relatively open shortgrass prairie. Massasaugas were active between 14 and 30degreesC, with an average ambient temperature during activity of 22.1 +/- 2.5degreesC. The time of observed activity shifted through the season, with primarily diurnal activity during the cooler months (April and late September to October) and primarily early evening activity during the hotter months (May through August). Massasaugas in Colorado gave birth to litters of 5 to 7 young between late August and late September, and reproduction appeared to be biennial. Populations of S. c. edwardsii in Colorado are scattered but population size appeared to be relatively large based on number of captures and low recapture rates, particularly in Lincoln County. However, due to habitat loss and fragmentation resulting from agricultural expansion, these populations might become increasingly threatened in the future.