A study was made of the effect of salinities of 0, 3, 6, 9, 12, 17, 22 and 27%o on the oxygen consumption and osmoregulation of Macrobrachium tenellum. The metabolism salinity curve is "U" shaped although in freshwater the metabolic rate reduces by 38%. The oxygen consumption observed over this range of salinities corresponds to Type II of Kinne (1971). In M. tenellum, osmotic concentration of haemolymph remains constant around 531 mmol Kg -1 which means that this species is a strong hyper-hypo osmoregulator. The isosmotic point was 533 mmol Kg -1 which suggests that this species has only recently invaded the freshwater environment.

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