The female genital structures of nineteen species of the subfamily Paradiaptominae (Diaptomoidea, Diaptomidae) were studied using light and scanning electron microscopy (SEM). They exhibit a common organization characterized by the absence of seminal receptacles and by the presence of a ventral genital operculum covering the paired gonopores, confirming the general pattern of the Diaptomoidea. The taxonomic value of these structures is analyzed: the shape, size and position of the genital operculum on the genital somite is proposed as a new diagnostic character for the subfamily. The Paradiaptominae species are characterized by a peculiar placement of the spermatophore on the female, in dorsal or laterodorsal position on the urosome. The observations lead to a new hypothesis on the mode of fertilization of the females via a fertilization tube.