Growth rates and energy bodgets were determined for three size groups of grass carp (Ctenoharyngodon idella Val.) fed ad libitum rations of lettuce leaves at 30 degree C. Mean initial sizes of the fish were 12.8g, 37.2g and 95.2g, respectively. Growth rates in terms of wet weight and protein decreased with increasing body size, but there were no significant differences in growth rate in terms of dry weight or energy among size groups. Body size did not significantly affect the rate of food consumption per unit weight or proportions of food energy channelled to each component of the energy bodget. The average energy bodget was: 100C= 23F + 3U + 53R + 21R. where C is food consumption, F is faecal production, U is nitrogenous excretion, R is metabolism and G is growth. It was concluded that the decline in growth rate in terms of wet weight with increasing body size was mainly caused by an increase in the final energy content per unit weight of fish.