The Eastern Massasauga rattlesnake, Sistrurus catenatus catenatus, is a candidate for US federal listing and is legally protected in every state or province in which it occurs. Habitat degradation and human persecution have contributed to range-wide population declines. Survival estimates are essential for a thorough understanding of population dynamics, yet are rarely reported for S. c. catenatus in the peer-reviewed literature. There has been little research on massasauga survival in managed areas of Michigan, USA, the state considered to be the last S. c. catenatus stronghold. Our objectives were to estimate survival of massasaugas during the active season (May–October) in southwestern Lower Michigan and describe causes of mortality. We captured (mid-May to late August), radiomarked, and monitored 27 adult massasaugas in 2008 and 2009 and pooled data for analyses. We observed snakes throughout the active season and estimated survival (Mayfield method) for that period (11 May–29 October; 168 d). Cause-specific mortality was investigated qualitatively. Estimated survival probability for the active season was 0.9472 (CI  =  0.8518–1.0000), higher than any estimate for similar studies of which we are aware. The single mortality event observed was caused by predation. We suggest that the high massasauga survivorship in this area might be attributable to habitat management for S. c. catenatus combined with a relative lack of infrastructure and human persecution. Our results suggest that adult S. c. catenatus survival is potentially geographically variable and can be high in areas not well-investigated, such as southwestern Lower Michigan. More research would yield long-term survival trends for the studied population as well as throughout their range.