Four groups of 18 beef calves each were used to evaluate effects of different treatments on parasite control and weight gains. The investigation extended from November 1986 (weaning) to October 1987. Group-1 calves were treated with ivermectin (200 micrograms/kg of body weight, SC) at approximately 6-week intervals for a total of 8 treatments; group-2 calves were given the same dosage of ivermectin by the same route of administration as group-1 calves in November, March, and July; group-3 calves were given fenbendazole paste (5 mg/kg, PO) at the same times as group-2 calves; and group-4 calves served as untreated controls with provision for ivermectin salvage treatment. All groups grazed on individual pairs of larval-contaminated, 1.6-ha pastures. Highest (P less than 0.05) initial worm counts in fall tracer calves were found in group 3 (Ostertagia ostertagi and Trichostrongylus axei adults) and group 4 (O ostertagi and Haemonchus adults). Fecal egg counts of group-1 calves were low throughout the experiment and pasture larval counts remained negligible after July. Egg counts and larval counts of other groups remained higher into summer. Worm counts, including O ostertagi inhibited early fourth-stage larvae (EL4), were highest (P less than 0.05) in groups-3 and -4 spring tracer calves; numbers of O ostertagi EL4 were similarly high in groups 2, 3, and 4; and T axei counts were highest (P less than 0.05) in groups-3 and -4 yearlings slaughtered in spring. Liveweights of group-1 calves were greater (P less than 0.05) than in other groups from March 2 to October, and by July 2, group-2 calves had a liveweight advantage over group-4 calves. Group-3 calves had the lowest rate of gain from March to July and mean liveweight of the group was less (P less than 0.05) than in all other groups from April to October. Only minimal worm numbers were recovered from groups-1 or -2 calves in October. Large numbers of O ostertagi and T axei were recovered from group-4 calves and O ostertagi from group-3 calves. A few calves in groups 3 and 4, but particularly in group 4, were affected by type-II disease (chronic to acute gastritis caused by maturation and emergence of previously inhibited larvae) from August to October. Final mean liveweights in descending order were 365 kg in group 1, 328 kg in group 2, 316 kg in group 4, and 281 kg in group 3.