Gonadal condition of hard clams (Mercenaria mercenaria) planted at 2 tidal locations and at 3 population densities were evaluated in relation to age, size, sex, season, and culture condition. Changes in gonadal-somatic indices (GSI) reflected seasonal differences in gonadal development. Similar decreases in GSI were observed during the spring (May-Jun) and fall (Sep-Oct) spawning peaks. GSI varied significantly (P < 0.0001) with clam size and age. Larger clams had proportionally more gonadal tissue than smaller clams of the same age. Similarly, older clams had larger GSI than younger clams of the same size. No statistical difference (P> 0.05) was detected between the GSI of female and male clams of the same age and size. Clams grown at the lowest density level or at the subtidal location were larger and had proportionally more gonadal tissue than clams from higher densities or the intertidal location. Differences in size of clams among treatments explained only the variation in GSI among density treatments, but not between tidal locations.