Regular post-mortem counts from 'tracer' and flock sheep, together with faecal worm egg counts and plasma pepsinogen determinations, formed the basis of an epizootiological study of trichostrongylid infections of Merino sheep in the Western District of Victoria. Species from genera Trichostrongylus, Ostertagia and Nematodirus predominated in the worm counts; Haemonchus contortus was absent. Counts of Ostertagia and Trichostrongylus spp. from successive groups of 'tracer' sheep showed a pronounced seasonal distribution of available larvae in accordance with the temperature and humidity components of the prevailing weather. Mean worm counts ranging from 2000 to 32,000 per fortnight were recorded between May and October, whereas at other times the counts were consistently less than 500. When the availability of larvae was high, faecal worm egg counts of weaners and adult sheep were less than 150 epg. At this time the majority of the worms in the abomasum were early fourth stage larvae. Some weeks after availability had decreased to low levels, mean egg counts increased from 100 to values over 700 epg. It was concluded that the high levels of larval availability were derived from worm eggs deposited in the previous summer and autumn. Resistance to the establishment of worms was evident in both young and mature sheep. It was found that raised plasma pepsinogen levels, indicative of severe abomasal lesions, were closely related to the numbers of newly ingested larvae. Liveweights and fleece weights of weaners treated with 88 mg thiabendazole per kg every 14 days and those from untreated weaners did not differ significantly until the availability of larvae decreased to low levels, after which the treated sheep were heavier and produced 0.52 kg or 14.6% more wool per head.
