Twenty-five patients with the clinically established and laboratorily verified diagnosis of urogenital trichomoniasis were examined. Transmission electron microscopy was used to study the ultrastructure of cultured Trichomonas vaginalis (control) and its change induced by the mutagen nitrosoguanidine. It was found that pino- and phagocytosis was activated by the mutagen and a lot of electron-dense minor hydrogenosomes appeared in the cytoplasm of protozoa. It was suggested that there was an association of the found changes with the activation of metabolic processes requiring enhanced energy provision. No mutagen-induced change in the morphological pattern of protozoa was detected. The heterogeneous forms of Trichomonas were observed in the control and experiment.