Abstract
Epidemiological studies over a seven year period in the non seasonal rainfall zone in Australia showed a biphasic pattern in intestinal Trichostrongylus spp worm burdens of tracer lambs, with T colubriformis predominating in the autumn peak with T vitrinus in the spring peak. Inhibition of these species was recorded which was not obviously associated with the size of the worm burden, developing resistance of the host or environmental conditions experienced by the free living stages. Possible seasons for the seasonal change in the incidence of the two species and occurrence of inhibition are discussed.