The fine structure of Trichomonas vaginalis and its acid phosphatase activity in the vaginal medium was investigated with the aid of the electron microscope. The relationship among trichomonads, desquamated epithelial cells, macrophages and bacteria was examined with special attention directed towards their phagocytic behavior. Acid phosphatase activity was observed in the Golgi apparatus, phagolysosomes and in the chromatic granules of Trichomonas vaginalis. Macrophages and leukocytes also exhibited activity for acid phosphatase, particularly during phagocytosis. Phagocytosis and digestion of epithelial cells and bacteria by trichomonads was demonstrated. The study of trichomonads in the vaginal medium is considered to be an important approach towards the understanding of the pathogenesis of lesions induced by this parasite.