Effects of different daily rations of the oligochaete wormTubifex tubifex on survival, moulting and food conversion ofMacrobrachium lanchesteri (Palaemonidae) andCaridina weberi (Atyidae) have been described. Moulting is a metabolic necessity for either species and occurs even at the expense of organic reserves of starving prawns. The geometric derivation of the growth-feeding rate relations inM. Lanchesteri indicated that, 88, 162 and 204 mg live food/g live prawn/dav represent the maintenance, optimum and maximum feeding rates respectively. The corresponding values forC. Weberi were 96, 236 and 396 mg. Food availability has a marked influence on the food partitioning and biochemical composition of either species.