Morphogenesis of the genital system of 14 species of nematodes of the superfamily Tylenchoidea was studied. The evolution of the genital system of females in the superfamily was considered. The initial type for the group is a paired gonade of the females. In different families the reduction of the hind branch of the gonade proceeds in parallel. During ontogenesis the development of the posterior part of the trophicogenital department and the gonade branch restrains. In the family Pratylenchidae this phenomenon is associated with the shortening of the body during the transition to endoparasitism; in the family Tylenchidae the phenomenon is connected with the acquiring of the streamline form suitable for living in humid biotopes. In the family Anguinidae takes place the hypertrophy of the female genital system connected with the transition to the cavitary endoparasitism. A new technique for making preparations of nematodes is suggested.