AbstractIt depends on the trophic position of a species whether its growth is limited by the quantity or quality of diet: predators are frequently limited by the quantity of diet, primary consumers frequently by the quality of diet. The protein deficiency is of decisive importance for the growth of primary consumers. The digestible protein/digestible energy ratio is most important: 4 mg/kJ are sufficient for maintenance, 20… 30 mg/kJ are necessary for maximum growth. These values can be confirmed by feeding tests withCarassius auratus. According to measurements by different methods, the organic nitrogen is present in the detritus in the form of amino acids as bonded to organis matter. Thus,Sarotherodon mossambicusassimilates 25.6 mg amino acids/kJ from detritus, whereas only 25% of these amino acids can be identified as protein. Due to the utilization of this source of diet, detrivorous fish occupy a special position in the chain of diet, thus gaining a great importance for the aquatic cultur