In their natural habitat, brown-striped frog ( Limnodynastes peronii) larvae periodically swim rapidly from the bottom of their ponds to the water surface and then immediately dive to the bottom again. This behaviour is presumably related to air-breathing. We examined the behavioural and metabolic responses to aquatic hypoxia in L. pernoii larvae. Gas filled lungs were found in all free-swimming larval stages of L. peronii, but air-breathing occurred infrequently in normoxic water. The frequency of air-breathing at 30°C increased rapidly in hypoxic water when oxygen partial pressure ( P o 2 ) fell below 10 kPa. Only a slight increase was observed at similar oxygen partial pressures at 20°C. The critical oxygen tension at 30°C was about 7kPa, below which, aquatic breathing larvae become metabolic oxygen conformers. In natural habitats where surfacing behaviour was observed, temperatures during summer months frequently exceed 25°C and some ponds become extremely hypoxic ( po 2 < 3.0 kPa); therefore air-breathing appears to be the only way in which these larvae can maintain a fully aerobic metabolism.