Sets of tracer calves, which were exposed for 2 months on irrigated pasture and then slaughtered, provided evidence of the extent of seasonal helminth infestation. Haemonchus spp. were the most abundant nematodes, the largest numbers of which were generally recovered from May-August. Marked inhibition of development in the 4th larval stage was noted from May-November. Trichostrongylus spp. were recovered in modest numbers with the highest burdens generally present in May, June and October. In the first year of the survey Cooperia spp. were recovered in small numbers, but in the second year burdens increased from February-June.