The reproductive cycle of Eurhomalea exalbida (Chemnitz, 1795) in Ushuaia Bay, Beagle Channel, was studied from October 1998 to December 1999. Clams were collected monthly by SCUBA diving at 3–5 m water depth. The degree of sexual maturity was determined histologically (n=338). The sex ratio was 1:1 (Chi-squared p>0.05). First sexual maturity occurred at 39 mm shell height in males and 40 mm in females (i.e., 4 years of age). Five distinct histological stages of sexual maturity could be established in males: (1) early active, (2) late active, (3) ripe, (4) partially spawned with recovery and (5) spent. Females had oocytes ranging from small and immature to large and fully developed in their ovaries throughout the year. Since we could not classify reproductive stages based on a qualitative histological scale of sexual maturity, quantitative measures using an image analyser were used. In males a high percentage of mature individuals were present between January and August. The percentage of early active and partially spawned individuals was highest in November 1998 (70%) and 1999 (60%). In females, the mean number of developed oocytes per unit of gonadal area in a cross section, the mean diameter (minor axis) of oocytes, and the percentage of gonadal area occupied by oocytes were lowest in November, indicating a spawning peak. These results indicate synchronous gonadal development and spawning in males and females. The more intensive spawning activity in November coincides with the higher biomass and production of phytoplankton in spring. © 2002 Taylor & Francis Group, LLC.