Juvenile Sarotherodon mossambicus in Lake Sibaya, South Africa, grow rapidly and maintain good body condition, but adults grow slowly, show extensive marasmas and appear to suffer from severe malnutrition. The cause of this malnutrition was sought by comparison of juvenile and adult diets. Juveniles feed on benthic detrital aggregate from nearshore sand terraces, whereas adults feed on benthic detrital aggregate in deeper offshore waters. Chemical analysis of 39 benthic detrital aggregate samples collected from juvenile and adult feeding areas showed that detrital aggregate from these 2 areas did not differ in caloric content (° =0.97 J/mg organic), organic content (° = 43.0%), soluble carbohydrate content (° = 16.0% of organic), total carbohydratecontent (° = 45.7% of organic). Protein content ranged from 1.8 to 14.2% of organic matter and decreased with increasing depth. Assimilation efficiencies determined experimentally for juveniles feeding on benthic detrital aggregate were: energy 42%, organic matter 37%, soluble carbohydrate 36%, total carbohydrate 35%, and protein 46%. A large proportion of the organic matter assimilated was nonliving amorphous detritus. Ratios of digestible protein to digestible energy calculated for the 39 samples show a highly significant inverse relationship to increasing depth. These ratios indicate that juvenile diets contain adequate protein to support good growth, but adult diets would be expected to result in malnutrition due to protein deficiency. Nutritional constraints imposed by low protein levels in detrital aggregate account for the characteristic nearshore distribution of detritivorous fishes and the paucity of exclusively detritivorous fish species.