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We studied Desert Massasauga Rattlesnakes (Sistrurus catenatus edwardsii) in southeastern Colorado in 1998 and 2005-2007. Mark-recapture data for 770 snakes indicated a population size of > 3,500 snakes in an area of approximately 4,800 ha. Field growth rates and size class frequency distributions showed that average snake age was 3 yr; 4 yr old snakes were frequently encountered, but less than 4% were 5 yr or older, suggesting low survivorship beyond this age. Conversely, initial growth was rapid; snakes grew an average of 0.57 mm/day in their first full year. Desert Massasaugas mated in fall and spring, producing 2-7 (mean 3.3) young in late August to early September, and reproduction appeared to be biennial. Desert Massasaugas showed maternal attendance for at least 5 days postparturition, and neonate dispersal corresponded with the first shed. Radioed Desert Massasaugas (N = 15) used rodent burrows as hibernacula, and within 50 m of Desert Massasauga hibernacula, eight snake, five anuran, and two turtle species use the same area for hibernation. Low prey density at the hibernaculum indicates that stable hibernation conditions are the primary resource attracting a diverse assemblage of species to this area. The hibernaculum area serves as a critically important winter refuge for numerous species and supports the largest known population of Desert Massasaugas. This population is considered stable at present; however, because of rapidly changing climatic conditions, habitat loss and degradation, anthropogenic disturbance, and shifts in prey abundance, it may become threatened in the near future, and continued monitoring is warranted.