Mebendazole was given to mice infected with Angiostrongylus costaricensis at a single dose of 5 mg/kg at 6, 11, 16 or 21 days post-infection (p.i.) and in five successive doses at 5 mg/kg daily at 6, 11 or 16 days p.i. The effects were comparatively assessed by examining various parameters in host mice and worms. As a whole, the effects of mebendazole were caused more conspicuously by five successive treatments than by a single treatment. In both treatment modalities, the effects were more remarkable in earlier treatments, and nearly complete effects were caused by five successive treatments before 15 days p.i. These results suggest that the inhibition of egg formation and/or oviposition will inhibit the pathological changes caused in the disease by A. costaricensis, especially before the onset of the changes.