Freshly laid eggs of Chelodina rugosa survived for up to 12 weeks when immersed in water and subsequently underwent successful incubation and normal hatching. Embryonic development was arrested during immersion, remained arrested in an atmosphere of nitrogen, and recommenced when eggs were exposed to air. The hypoxic conditions during immersion appear to extend the arrest typical of turtle embryos during their period in the oviducts. Freshly laid eggs of the temperate.zone C. longicollis died when immersed for longer than one week and eggs of both species died when immersed after post.laying embryonic development had commenced. These results, supported by anecdoctal and experimental evidence, suggest that C. rugosa lays its eggs in saturated or flooded ground in the late wet or early dry monsoonal season. Embryonic development presumably remains arrested until water levels drop and oxygen tensions in the nest rise by diffusion through the drying soil. Partly developed embryos in nests that are flooded after laying would perish. In contrast, C. longicollis of temperate Australia nests only in relatively dry substrates, and its eggs appear not be have evolved the capacity to withstand immersion.