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The life cycles and ecology of two closely related species of lampreys of the genus Mordacia were studied in various rivers in south-eastern Australia. In the Moruya River in southern New South Wales, the anadromous parasitic species, M. mordax (Richardson), has a larval life of three and a half years with metamorphosis occurring in late February or early March. The nonparasitic species, M. praecox Potter, metamorphoses at a different time (late October or November) and at a significantly longer length than M. mordax. The latter probably reflects a longer larval life in M. praecox, an estimate of which could not be obtained from the available data. Thus an extension of the larval phase, as well as a reduction in the post-metamorphic stages, may have occurred in the evolution of nonparasitic lampreys from parasitic forms. Evidence is presented to show that seasonal differences in temperature influence the pattern of ammocoete growth throughout the year, while local differences in the density of food are responsible for differences in growth rates in different areas. The effect of flooding and reduction in the water levels on the distribution of ammocoetes is discussed. A description is given of the behaviour of metamorphosing and transformed M. mordax prior to their migration out to sea. The above data is collated with that obtained from other studies to produce an overall picture of the life cycles of the two species of Mordacia.