The epidemiology of Ostertagia ostertagi and other gastrointestinal nematodes in yearling beef cattle was examined in each of 2 successive years. During each year 50 head of newly weaned beef calves were given a single dose of thiabendazole and then placed on experimental pastures. Twenty-four of the animals were designated for monthly slaughter (n = 2) and analysis of worm population characteristics and 25 were designated for blood and faecal collection and for weighing. Parasite-free tracer calves were grazed alongside the yearling cattle each month (n = 2) throughout the 2 years and were also slaughtered for analysis of worm populations. Faecal egg counts, plasma pepsinogen determinations, herbage larval counts, and animal liveweight changes were recorded monthly. Results of this work substantiated previous observations on seasonal changes of populations of the different nematode genera, but greater continuity and definition of patterns was possible in the present work. Ostertagia ostertagi was the predominant nematode present. While minimal numbers of inhibited larvae were observed from autumn into winter, most of the population was adult at this time. Acquisition of inhibition-prone larvae begins in late winter and peak numbers are acquired between March and June. Little transmission of O. ostertagi occurs between June and September. Trichostrongylus axei had a similar seasonal prevalence to O. ostertagi. A major difference in the 2 years was in the pattern of maturation of inhibited larvae. This occurred early, during May 1981, following below average temperatures in May and above average rainfall in May and June. Overt type II disease was not associated with a smaller level of maturation in autumn 1981. In contrast, maturation of large burdens of inhibited larvae did not occur until autumn 1982 following a protracted period of dry and hot weather from spring. Acute type II disease was observed in autumn 1982. Reasons for the differences in maturation pattern, based on worm counts from yearling cattle and tracer calves and association of these with faecal egg counts, herbage larval counts, clinical condition, and liveweight changes are discussed.