Yearling ayu <i>Plecoglossus altivelis</i> collected in June and July from the Ohta River were biologically and biochemically examined in comparison with annual ayu caught in the same area.<br>All the yearling ayu were female. The yearlng ayu were different in size and skin color from the annual ayu, but not different in anatomical parameters such as hepatosomatic index, intraperitoneal fat body ratio, and gonadosomatic index. The ovaries of annual ayu were immature and included chromatin nucleolus and perinucleolus stage eggs, but those of yearling ayu consisted mostly of abortive eggs and a few chromatin nucleolus and perinucleolus stage eggs.<br>The yearling ayu had less muscle protein but more triglycerides than annual ayu. Fatty acid composition of muscle triglycerides in the yearling ayu was not different from that of the annual ayu. Low protein accumulation caused by depression of protein metabolism would affect the viability of yearling ayu after July.