The femoral arteries of newborn mice which were experimentally infected with encephalomyocarditis (EMC) virus were studied electron microscopically. Viral crystalline aggregates were observed within the cytoplasm of adventitial fibroblasts in 4 of 5 animals studied and within a macrophage of one animal. The nuclear chromatin of medial smooth muscle cells was frequently marginated, and margination or pyknosis of nuclear material was observed in fibroblasts of the tunica adventitia.