In newborn mice infected subcutaneously with the NB5092 strain of Ross River virus there was early and extensive growth of virus in extraneural tissues, especially striate muscle, brown fat, and smooth muscle. Several days after peak titers had been attained in muscle, infected mice developed widespread muscle necrosis with paralysis of hind limbs, but recovery was complete by 35 days. In brown fat, lesions became calcified and were surrounded by giant cells; some lesions were not resolved by 60 days. The. T48 strain of Ross River virus, unlike the NB5092 strain, regularly infected the heart and caused myocardial necrosis, and later, calcification. Both strains infected perichondrium, periosteum, and skin. Semliki Forest virus infected, with acute necrotic changes, the same extraneural tissues of newborn mice as did Ross River virus, but the infection was generally less extensive.