The growth and cellular composition of the thymus, spleen and kidney were monitored in Rainbow trout, Salmo gairdneri, from 1 to 15 months of age. All organs grew as the fish grew, but they attained their maximum relative weights (expressed as a percentage of body weight) at 2–3 months ofage. The organ weights showed a closer correlation to body weight than they did to age. The total number of leucocytes in the lymphoid organs increased with age; however, the number per gram of lymphoid organ tissue, and per gram of tissue per gram of fish, decreased with age. The number of cells present showed a closer correlation to the weight of their respective organ, than they did to age. A histological study of the lymphoid organs showed gradual changes in the structure of the lymphoid tissues. The thymus showed some signs of involution even at 15 months of age. The spleen and kidney had mixed populations of erythrocytes and leucocytes, with a gradual increase in the amount of melanin as the fish aged. The kidney in particular had a rich variety of leucocytes. The blood picture showed no obvious fluctuations with age or season over the period studied, with the proportion of lymphocytes present remaining remarkably constant.