The results of an epidemiological study involving the estimation of worm counts of gastro-intestinal nematodes of lambs, during the period December 1978 to November 1979, in the Baghdad area are reported. Ostertagia spp. and Trichostrongylus spp. were the most abundant nematodes in the weaned lambs used in this study. Ostertagia spp. infection was present throughout the period of observation, the level of infection being the highest during early summer (May-June). Trichostrongylus spp. infection, however, reached a peak during July-August. In the tracer lambs the maximum population of Ostertagia spp. was found during February-March, while that of Trichostrongylus spp. could be observed only after mid-June. Other nematodes encountered were Haemonchus contortus and Trichuris sp. but they were only present in small numbers. The proportion of inhibited larvae of Ostertagia spp. was markedly high during the dry summer months. Little inhibition was noticed during autumn and the early winter months. It appears that if the seasonal inhibition of Ostertagia spp. in Iraq is brought about by an environmental stimulus acting upon pre-parasitic larval stages, that stimulus cannot be chilling or falling temperatures, as observed in temperate northern zones.