The poison fangs vary in length from 1.3 per cent of snout-vent length in juveniles to 1.0 per cent in large adults. Fangs of captive cottonmouths were shed and replaced at intervals of about 21 days, but the interval was variable. Relationships in distance between the base of fangs and between fang punctures in an actual bite indicate that examination of the wound does not provide a good basis for judging accurately the size of the snake that inflicted the bite. In general, females less than 450 millimeters in snout-vent length were juveniles; those more than 450 millimeters were classified as post partum or reproductive on the basis of sizes of ovarian follicles. Since about half the adult females were fecund, it was concluded that a biennial reproductive cycle occurs in this species. An annual cycle may occur in areas where temperature permits year-round activity. It was estimated that females become sexually mature at an age of approximately two and one-half years. Mating is probably most concentrated in early spring at about the time when females ovulate, but copulation is not a stimulus for ovulation. Sperm retention and delayed fertilization allow young to be produced without copulation occurring in each breeding season. The testes increase in size gradually rather than rapidly at maturity or in each breeding season, but seasonal cycles in sperm production occur. The gestation period is three and one-half to four months. Determination of sex in the embryos is possible by late June, because the hemipenes of males are evaginated until the time of birth. Parturition generally occurs in August or September, but captivity may delay birth for a month or more. From one to 16 young per litter are born, depending on size of the mother and other factors; but the average is between six and seven. Mortality rate at birth is high in captive individuals but has not been determined in natural populations. The sex ratio in embryos and adults examined revealed about 53 per cent females. Because sufficient information on population composition is not available, an estimate of the percentage of adults in a natural population was based upon the number found in my study. The reproductive potential was estimated from these figures. Normal young at birth are 230 to 240 millimeters in snout-vent length, but their size is influenced by the condition of the mother. Comparison of newborn young with those captured in spring indicates that little growth occurs during winter. Early growth is largely dependent upon feeding before winter quiescence. The umbilical cord is broken at birth and the navel closes within a few days, but the scar remains throughout life. Sexual dimorphism in the position of the scar is characteristic of some snakes but is minimal in cottonmouths.