Abstract. Semelparity is one of the most drastic reproductive strategies found among marine invertebrates. It is frequently found in species whose members have small adult sizes and brood embryos internally. In this study, we describe the reproductive strategy of the bivalve Gaimardia bahamondei to explore the possible causes of the association between semelparity and internal brooding. Males of this species exhibit continuous gonadal activity throughout the breeding season. Apparently continuous spawning of sperm is associated with an abundance of captured sperm in the adfrontal region of the gill filaments of both males and females. Females are capable of brooding three cohorts of embryos simultaneously while also producing three new cohorts of oocytes. This suggests that females are able to generate at least six cohorts of embryos during the breeding season. Embryos are brooded in the suprabranchial chamber and are individually surrounded by a membrane with a projecting peduncle; embryos are anchored by this peduncle to the adfrontal region of the gill filaments. Members of the youngest cohort differ in size, color, and shell ornamentation from members of the two older cohorts. There is no difference between members of the two older cohorts in size, but there is with respect to coloring and shell ornamentation. The importance of the embryonic cohorts in terms of their percentage of the total number of embryos varied among brooding females, suggesting among‐female variation in the timing of release of the oldest cohort of embryos. Members of this cohort break loose from the gills, lose their surrounding membrane, and fall into the ventral region of the suprabranchial chamber, from which they are evacuated to the exterior. Continuous sperm production in males and the production of at least six cohorts of embryos in females suggest that the costs of reproduction are high, which may partly explain the semelparity identified in this species.