A scanning electron microscope study of Hypotrichomonas acosta (Moskowitz), Trichomonas vaginalis Donne, Pentatrichomonas hominis (Davaine) and Tritrichomonas foetus (Riedmuller) provided new information about the structure of the periflagellar canal; emergence of the flagella from the cell body; structure of the undulating membrane; and position, shape and size of the pelta. Of special interest were the spatial relationships of the attached part of the recurrent flagellum and the accessory filament in Hypotrichomonas and in the members of Trichomonadinae, i.e., Trichomonas and Pentatrichomonas.