Abstract: The thymus of grass carp is situated on either side of the upper corner of the opercular cavity. It lies closely under the membrance of the opercular cavity. The projecting part of the thymus is protracted into the subtemporal hollow. The shape of the total thymus is similar to that of water chestnut. The thymus of grass carp consists of the outer, middle, and inner regions. The middle and inner regions consist chiefly of lymphocytes and reticuloepithelial cells, whose structural features are similar to those of the cortex and medullary region of the higher vertebrates respectively. There is no abvious demarcation between middle and inner regions. There are three types of lymphocytes in thpmus: large (4%), medium (18%) and small lymphocytes (78%). The number of thymus lymphocytes is approximately 3.6?06 cells per milligram in oneyear-old fish, and 2?06 cells per mg in two-year-old grass carp. In one and two year old grass carp total weight of the thymus increased with age, but in bigger fish thymus weight does not change in a regular way. Adult fish show apparent degeneration in thymus. The thymus of grass carp also shows age-independent degeneration caused by nutritional and enviromental factors.