Paracerceis sculpta (Holmes) is a sexually dimorphic sphaeromatid isopod crustacean inhabiting intertidal zones in the northern Gulf of California. Adult females in this species undergo a biphasic moult that initiates their sexual receptivity. Coincident with this moult, females experience anatomical changes associated with oviposition and gestation of young within the female marsupium. These modifications restrict female mobility, prevent females from feeding, and ultimately result in obligate female semelparity. In this paper, I describe the external and internal anatomy of premoult, half-moulted and postmoult females collected from breeding aggregations in intertidal sponges. Field-collected premoult females possessed hard, lustrous cuticles, and did not contain sperm. Females in half-moulted condition contained sperm masses in both oviducts. Postmoult females contained developing embryos in internal brood pouches, and possessed cuticles that were pliable, translucent and setose. Embryos completed their development within postmoult females and dispersed from their mothers as mancae. Gestation duration decreased with increasing ambient temperature, from a maximum of 75 days at 12 dGC to a minimum of 13 days at 35 dGC. Female fecundity correlated positively with female body length. Spent females, depleted of lipid stores and muscle mass, died within two weeks of releasing mancae. ‘Female metamorphosis’ is considered a taxonomic character in certain sphaeromatid species descriptions. Anatomical changes in reproductive females similar to those in P. sculpta may occur in these species as well.