Factors affecting broodstock conditioning in the hermaphroditic basket cockle, Clinocardium nuttallii, were examined in laboratory experiments. Although a 13-wk experiment conducted at 16°C failed to bring C. nuttallii broodstock into spawning condition under different dietary treatments, initiation of gametogenesis was apparent for those broodstock maintained at 2.5°C. Condition indices were significantly greater when broodstock were fed a combination of Isochrysis galbana (Tahitian strain, T-iso) and Chaetoceros gracilis (CG) or T-iso and Thallasiosira pseudonana (3H) than when fed a combination of Tetraselmis suecica (TS) and 3H or TS and CG, indicating that T-iso may be a better food source than TS for cockle broodstock conditioning.