In order to clarify the structure of the male genitalia and establish a new parameter to be used in taxonomy, it was made a study of the morphology of Limnogonus aduncus Drake & Harris, 1932 - Gerrinae and Halobatopsis platensis (Berg, 1879) - Trepobatinae. Eleven structures were observed in the phallus. A clear difference was found between tory apparatus, vesica, phallosoma, struts, conjunctiva and parameres.