Seasonal content changes of the three key nutrients for organisms, protein, lipid and glycogen, were analyzed for a whole year to delineate the seasonal energy strategy in pen shells, Atrina pectinata. Two metabolically important organs, the visceral mass and the posterior adductor muscle, were examined. Protein in the visceral mass rose in April and stayed at the level until June followed by the constant minimum value between August and November. The protein contents in the posterior adductor muscle increased sharply in April and again in July, followed by a gradual decline thereafter. Total lipid contents in the visceral mass gradually increased between January and May, and then slowly decreased until September since which a new weak increase was noticed. Lipid levels in the adductor muscle rapidly dropped in June and July. Glycogen contents in the visceral mass rapidly increased between February and June, followed by a drastic drop in July. This reduced visceral glycogen level was maintained up to September, and a gradual reduction ensued. Glycogen contents in the adductor muscle steadily but markedly increased from April reaching the maximum in August, and then slowly declined thereafter. These results suggest that an accelerated protein and lipid synthesis occurs in the gonad when the pen shell undergoes the ripe stage of gametogenesis, but the levels of these two nutrients decrease on spawning. With this gonadal process, regular protein synthesis and lipid storage in the posterior adductor muscle are temporarily arrested. The most important nutrient reserves that support gonad developmental cycles in a long term seem to be glycogen of the posterior adductor muscle.