We conducted a nine-year mark-recapture study of a population of Crotalus viridis oreganus at a hibernaculum in north central Idaho from 1982 to 1990. Snakes were captured by hand and drift fence traps and individually marked. We made 627 recaptures from 319 marked snakes (176 males, 143 females). The primary sex ratio did not differ significantly from 1:1. Males were not significantly different in length or mass from females at birth. Estimated average individual growth rates in length beginning at birth (18.2 and 20.5 cm/yr for females and males, respectively) were not significantly different between sexes. However, males grew to a significantly longer estimated average asymptotic length (88.8 cm) than females (69.2 cm). The average annual growth rate in mass did not differ significantly between males and females (49 and 43 g/yr, respectively). The maximum snout– vent lengths of males and females in this study were 96.0 and 79.5 cm, respectively. Snakes underwent ecdysis two to three times during their first year and most shed two more times during their second year of growth. Most known-aged females reached sexual maturity in their fourth summer, but some males matured during their second summer. Clutch size varied from 3 to 8 young per female (mean = 4.8). The predominant female reproductive cycle was biennial. Three females showed an annual cycle, the first documentation of this in a temperate zone pitviper, but only for two successive years. Only one female had a confirmed triennial cycle, but we suspect that this was underestimated. The proportion of females in reproductive condition was significantly related to prey density of the prior year. Survival was highest for adults followed by immatures and newborn, and survival decreased for all age groups over the period of the study (from 82 percent to 55 percent for adults). The decrease in survival may be related to a handling or study effect.