Gonadal development and sex differentiation have been studied in larvae, metamorphosing individuals and young adults of the lamprey Mordacia mordax (Richardson), one of the two anadromous species of the southern hemisphere family Mordaciidae. The parasitic species is remarkable for the fact that its fecundity is so low that it is little greater than those of dwarf nonparasitic species. The relationship between the phasing of gonadal development, pattern of sex differentiation and fecundity in M. mordax is compared with those of nonparasitic and parasitic northern hemisphere lampreys (Petromyzontidae) and with that ofGeoiria australis, the sole representative of the other southern hemisphere family (Geotriidae). Up to lengths of 50 mm, the gonads have only small numbers of germ cells and are small and histologically undifferentiated. A marked increase in germ cell number occurs at body lengths of 50–70 mm, when in future ovaries, many secondary gonia enter meiotic prophase leading to cytoplasmic oocyte growth. Differentiation of the ovaries is generally completed at 90 mm, when they constitute just over half of all gonads. Throughout larval life and even during metamorphosis, the future male gonads remain small, histologically undifferentiated and indistinguishable from earlier undifferentiated stages. As in nonparasitic species, sex differentiation is characterized by the appearance in future male gonads of a high proportion of variable numbers of female orientated germ cells (in meiotic prophase or oocyte growth) destined to undergo atresia. In this respect, M. mordax is closer to the patterns seen in nonparasitic species, than to the more direct or determinate types of gonadal sex differentiation of the parasitic species with high fecundity, such as the southern hemisphere Geolria australis and similar holarctic species. It is suggested that variations in the nature and course of sex differentiation in lampreys may be dependent on initial sex differences in germ cell numbers and hence on the size and rate of growth of the gonad.