The growth of the lymphoid organs,such as head kidney,spleen and thymus was stueied in Larimichthys crocea,from hatching to 12 months of age.A histological and ultrastructural study was performed on the development of the head kidney,thymus and spleen aging from just hatching to the 12 months.Head kidney was first present on the 3^rd day after hatching(DAH).Primordial haemopoietic stem cells were first observed in the head kidney and rapidly differentiated into different cellular types.Progenitor spleen was present on the 4^th DAH,located close to the gut,soon becoming rich in blood capillaries,red blood cells and thrombocytes.The thymus occurred obviously on the 4^th DAH,located on either side of the upper corner of the opercular cavity,closely under the membrance of the opercuhr cavity.The thymus was the last lymphoid organ appearing but showed quick development.The thymus consisted of outer thymocytic and inner epithelioid zones.There was no obvious demarcation between them,but both zones were visible.Head kidney,spleen and thymus grew as the fish grew.The spleen weight showed a closer correlation to body weight than they did to age.But head kidney and thymus weight showed a closer correlation to age.The total number of leucocytes in the lymphoid organs increased with age.But the number per milligram of lymphoid organ didn’t show a closer correlation to age.The head kidney was more lymphoid than the spleen.The blood components had no obvious ralationship with age or season during the period studied.