The life history of N. helvetianus is direct. At optimum temperature (28-29[degree]C) infective larvae hatched from all eggs in 20 days. Following ingestion by a susceptible bovine, the infective larva exsheathed in the small intestine where the 3rd and 4th molts occur. By the 8th day, the larva was in the 4th stage and sex was readily distinguishable. By the 15th day, 86% of the nematodes were 5th stage young adults resembling the mature adults except for size and sexual development. Penetration of the intestinal wall was not noted at any stage in the cycle; however, the 5th stage young adults were in intimate contact with the intestinal mucosa, coiled among the villi. The prepatent period ranged from 21-26 days in calves 1-10 months old. The patent period ranged from 12-132 days and there appeared to be a correlation between age of animal and length of patency. Super-infection was established in a 3-month-old calf originally infected when 1 month old. When 6 months old, this calf was resistant to further infection.