An increase in dietary protein for larvae of the black carpet beetle, Attagenus megatoma, resulted in a decreased digestibility that was partly compensated for by substantial increases in both the gross and net efficiency of conversion of digested food into body substance. When the dietary amino acids were in a free form (not protein bound), the rate of food consumption was significantly lower; however, the greater ease of digestion and more efficient utilization of the amino acid diet resulted in a growth rate comparable to that observed on an all-casein diet. Maximum larval weight gain occurred on the stock culture medium, probably due to a combination of a faster rate of food consumption, ease of digestion, and more efficient utilization. Larvae preferentially selected protein from some of the diets, but when no selection was possible, the efficiency of N utilization was inversely proportional to the concentration of N in the diet. The concentrations of faecal uric acid decreased with an increase in dietary protein concentration, especially when expressed as a percentage of the soluble faecal nitrogen.