Fenbendazole, at low dosage levels, was evaluated for its efficacy against inhibited early fourth stage larvae of Ostertagia ostertagi in naturally infected, yearling beef cattle in April. The mean number of inhibited larvae in 10 untreated control cattle was 51,500. At a dosage level of 1 mg per kg (pelleted fenbendazole) per day in feed over five days in 10 cattle, the percentage reduction in comparison to controls was: adults, 85.9 per cent; developing stages, 79.0 per cent; inhibited early fourth stage, 47.6 per cent; other abomasal genera (Haemonchus species and Trichostrongylus axei, 100 per cent). At a dosage level of 5 mg per kg (10 per cent liquid suspension) administered as an oral drench to 10 cattle, the percentage reduction in comparison to controls was: adults, 96.9 per cent; developing stages, 82.9 per cent; inhibited early fourth stage, 74.7 per cent; other genera, 98.6 per cent. Efficacy against parasite genera of the intestinal tract (primarily Cooperia species) was in excess of 94 per cent at both dosage levels. Some variability of efficacy was observed, in that large numbers of surviving inhibited O ostertagi larvae were recovered from two or three animals in treated groups.