The effects of benzimidazole anthelmintics in murine strongyloidiasis were examined. Thiabendazole 50 mg/kg daily produced a 91% reduction in the numbers of Strongyloides ratti larvae in the feces. A similar suppression was seen when thiabendazole was given during the intestinal phase, but no effect was noted when the drug was administered during the phase of larval migration. Thiabendazole had no effect on larvae in the skin or lungs, did not inhibit maturation of worms, and did not expel adult worms from the gut, but did reduce fecundity of adult worms in the intestines by 84%. Mebendazole and cambendazole 50 mg/kg daily totally suppressed excretion of S. ratti in the feces. A similar suppression was seen when the two drugs were given during the phase of larval migration or during the intestinal phase. They had no effect on larvae in the skin, and the reduction in larval numbers in the lungs was not statistically significant. When given during the migratory phase and early intestinal phase, they reduced the numbers of fourth stage larvae recovered from the gut by 95%. Mebendazole and cambendazole totally eliminated intestinal adult worms. Dose response studies indicated that in terms of orally administered dose, cambendazole was 100-1,000 times more active than mebendazole. Thiabendazole and mebendazole had no significant effect on S. stercoralis larvae in the muscles. In contrast, cambendazole 50 mg/kg daily for 4 days eradicated S. stercoralis larvae from the muscles. It is concluded that cambendazole may have significant advantages over both thiabendazole and mebendazole in the treatment of strongyloidiasis.