This is the first study of seasonal changes in gonad development and tissue biochemical composition of the cockle Fulvia mutica in Cheonsu Bay off the west coast of Korea. Gametogenesis commenced in December when the surface water temperature was 9.1°C. In May, the first discharge of oocytes was observed, and spawning continued until October. The condition index (CI), a ratio of tissue dry weight to shell dry weight, increased gradually from late May to July and declined dramatically in August and September, indicating that active discharge of gametes occurred in the bay during this period. Histology and the CI suggested that F. mutica spawning in Cheonsu Bay is continuous from early summer to early fall, with two spawning peaks in June and July, and September and October. All cockles examined in November were sexually inactive. The relatively low CI recorded in late fall to mid winter (November to January) coincided with high levels of ash and low levels of lipids and carbohydrates in the tissue, whereas protein levels remained unchanged. The observed spawning pattern of F. mutica in Cheonsu Bay suggests that the currently established timing for fishing closure, from July to September, should be expanded from June to October to protect earlyspawning broodstocks. This change may secure recruitment and lead to a subsequent increase of the wild cockle population in the bay.