The changes in the weight of the organs and the body composition (proportions of moisture, proteins, fat, and ash) of four species of trout have been followed during normal growth and after retardation of growth by underfeeding. The eyes tend to continue growth when the fish are underfed. Retarding of growth results in loss of fat and protein accompanied by an increase in the ash content. Underfeeding also results in the loss of fat in trout livers, and in some instances this loss of fat is accompanied by an increase in the iodine number values.