Investigations have been carried out on the multimammate rat (Mastomys natalensis), orally infected with 1000 third stage larvae of Ancylostoma caninum per animal, to evaluate the larvicidal efficacy of anthelmintics in this paratenic host. The oral or subcutaneous administration of 19 anthelmintics and filaricides revealed good activity for levamisole, cambendazole, and mebendazole against 15-day-old Ancylostoma larvae which were located in the muscular tissue. Similarly, good results were obtained with levamisole and cambendazole against the migrating third stage larvae, by treatment two to six days after infection. The benzimidazole derivatives, thiabendazole, oxibendazole, parbendazole, and fenbendazole showed marked activity only at high dosage rates. The anthelmintics methyridine, amoscanate, pyrantel tartrate, morantel tartrate; the microfilaricidal organophosphates dichlorvos, fenthion, and haloxon; as well as the micro- or macrofilaricidal drugs diethylcarbamazine, nitrofurantoin, nifurtimox, suramin sodium, and thiacetarsamide sodium failed to show larvicidal activity even in high dosages. The average larval recovery rate after oral infection with 1000 third stage larvae, in untreated control animals, was 14.85%.