Continuous in vivo observations of the behaviour of multinucleate, multiflagellate forms of Trichomonas vaginalis have been made in a microchamber at 36 degrees C. It has been proven that these forms are initial forms of the multiplication of the parasite by cell division. Serial in vivo shots have been taken in this microchamber of the budding of a young flagellate from a multinucleate, multiflagellate form. In vivo observations have shown that these round forms are multiplicable, living cell forms of the parasite.