The antimicrosporidial activity of albendazole was tested on Nosema bombycis in vitro in Spodoptera frugiperda cells and in vivo in Heliocoverpa zea larvae and pupae. Significant reductions in the percentage of infected S. frugiperda cells were obtained using a concentration of 5.3 micrograms/ml albendazole in tissue culture medium but recrudescence occurred after the drug was withdrawn from the cultures. Significant reductions in the number of spores harvested from 6th-instar larvae or pupae were obtained when doses of 0.2 to 4.0 mg were incorporated into the diet but, with the lower doses, some resurgence of infection occurred in pupae after cessation of drug intake. Established infections were almost eliminated from 6th-instar larvae and pupae after consumption of 2 or 4 mg albendazole and infections were not established at all when 4 mg was consumed concurrently with the infective spores. Even at the highest dose albendazole had no deleterious effect on the growth and viability of H. zea. Clumped chromatin in the nuclei of meronts, revealed by electron microscopy, reflected the selective anti-tubulin activity of albendazole and there was massive disorganization of sporogonic development.