Studies on Nematodirus battus Crofton and Thomas, 1951, have shown a marked seasonality in the pattern of transmission (Thomas and Stevens, 1956, Veterinary Record 68: 471-475; Baxter, 1957, Veterinary Record 69: 1007-1009; Thomas, 1959, Parasitology 49: 387-410; Gibson, 1963, Research in Veterinary Science 4: 258268). The life cycle normally involves only 1 parasitic generation per year. The infective thirdstage larvae develop over the summer within eggs deposited on pasture (Gibson, 1958, Journal of Comparative Pathology 68: 338-344). Eggs are sensitized by low temperatures during fall and winter; larval hatching follows in the presence of adequate moisture and proper osmotic conditions as the temperature rises above 10 C (Thomas and Stevens, 1960, Parasitology 50: 31-49; Christie, 1962, Parasitology 52: 297-313; Parkin, 1976, Parasitology 73: 343-354). This set of conditions conducive to hatching is regularly present only during spring (Parkin, 1976, loc. cit.); thus, hatching of eggs is concentrated into a few weeks during this time. Outbreaks of clinical nematodiriasis are determined by the timing and magnitude of the seasonal hatch of larvae and the presence of young, susceptible lambs. On occasion, deviations from this typical pattern have been reported. Completion of the life cycle has been recorded during other times of the year, particularly in the fall (Borgsteede, 1983,