Ross River virus, an Australian group A arbovirus, was adapted by serial passage to cell cultures and to day old mice. The results of titrations in mice of different ages allowed the comparison of virulence between different stocks. Passage in cell cultures depressed the virulence of virus while passage in mice raised the level of virulence. Clones of original virus populations revealed heterogeneity with respect to virulence but none of the 41 clones was as highly virulent as virus passed 10 times in mice. Clones selected in sequence during serial passage in mice indicated that adaptation proceeded by the overgrowth of variants of increasingly higher virulence, and that clones from relatively hhighly passaged s strains were still heterogeneous in virulence.