Cytoenzymological changes were observed in parasitic ciliates (Nyctotheroides puytoraci) and flagellates (Opalina sudafricana and Protoopalina sp.) after injection of their host, Bufo regularis, with a single dose of 0.5 mg beta-naphthylamine (BNA) per toad. The experiment was carried out during the host's pre-breeding season. Trophozoites of the given parasites were examined 21 days after the injection. The localization of the mitochondria and the relative intensity of succinic dehydrogenase (SDH) and deoxyribonucleic acid (DNA) activity in the control and BNA-treated trophozoites were compared. The results could prove useful for screening the carcinogenicity of chemicals by means of these parasitic protozoans.