Japanese Mamushi (Gloydius blomhoffii) has long been exploited for its medicinal value. We studied this snake's reproductive ecology on Hokkaido and assessed its vulnerability. We marked 299 individuals from 1999–2002 and kept 29 pregnant females in captivity until parturition. Pregnant females aggregated at gestation sites during summer and fall until just before parturition. Behavior suggestive of mating seasons was evident mostly in August and September. As expected, there was an apparent sex difference in body size in adults but not in neonates: females were larger than males. Consistent with previous reports, both neonates and adults were larger in Hokkaido than in southern Japan. The smallest pregnant female measured 44.8 cm in SVL, which was larger than values reported from southern regions. Offspring sex ratio was statistically equal: 88 males to 105 females. Litter size averaged 6.6 offspring. Longer females tended to produce larger litters and longer neonates. The proportion of pregnant to non-pregnant females, coupled with mark–recapture data, suggested that Mamushi females skip one or more years between reproductive bouts. Relative litter mass (RCM) was large (83% on average), and postpartum body condition was lower, especially so in females with larger RCM. Direct human-caused mortality was highest for pregnant females (70.2%), followed by non-pregnant females and then males. High adult mortality, particularly of pregnant females, raises a concern about population persistence, because Mamushi exhibit a slow life history. The tendency of pregnant females to aggregate at predictable locations further makes Mamushi populations vulnerable to human killing and exploitation.