We studied the reproductive ecology of a population of Western Diamond-Backed Rattlesnakes (Crotalus atrox) in south-central Arizona for four active seasons using radiotelemetry and portable ultrasonography. Snakes mate in the spring and fall, and females undergo vitellogenesis exclusively in the spring, ovulate in the early summer, and give birth in the late summer. Although parturition occurs at the same time of year in all rattlesnake species studied, females of most species initiate vitellogenesis in the fall, and it is unusual for females to delay this process until the spring. No females gave birth more than once in this study, indicating that reproduction is less than annual. Litter sizes range from 2–7 neonates (mean = 4.5). The sex ratio of the neonates was approximately equal, but male neonates were longer in snout–vent length and heavier than female neonates. There was no significant relationship between maternal snout–vent length and clutch mass, number of neonates, mean neonate mass, or mean neonate snout–vent length. Maternal postparturient mass was positively correlated with mean neonate mass, but not the other variables.