We describe the reproductive biology of northern populations of Crotalus willardi using data gathered from four long-term field studies, histological examination of museum specimens, and literature records. Spermiogenesis occurs summer to early autumn. All male C. willardi examined contained sperm in the vas deferens (June–September) and had enlarged kidney sexual segments (June–October). Parturition occurs from late July through August (n = 17). Postpartum females appear to remain reproductively inactive prior to hibernation. Secondary yolk deposition (vitellogenesis) commences in the late spring or early summer, one or more years after parturition. Ovulation and fertilization probably occur early in the spring, followed by 4–5 months gestation. Copulation occurs in midsummer to early fall (n = 10). Repeated observations of marked females over several years, proportion of adult gravid females (60% Crotalus willardi obscurus, 40% Crotalus willardi willardi, 51% pooled), observations on captive animals, and histological observations collectively suggest frequency of reproduction is facultative but typically biennial or longer in wild snakes. Litter size averaged 5.4 ± 1.6 SD (n = 25, range 2–9). Regression analysis of 20 litters revealed an allometric relationship where litter size increased as an approximately cubic function of maternal SVL (litter size = e^−14.64 SVL^2.66). Forty-two neonates measured 167.2 ± 7.4 mm SD (range 150–182) and 43 neonates weighed 6.8 ± 1.2 g SD (range 5.0–9.0). The shortest reproductively active specimens measured 406 mm (male) and 402 mm (female) SVL.