The rectal content of an apparently normal 1-week-old broiler chick yielded an unclassified cytopathogenic virus with cytopathic effects of the round type. It was identified as a picornavirus from the following: ribonucleic acid in the viral core; virus growth in the cytoplasm; a particle about 30 nm in diameter; resistance to ethyl ether, chloroform, trypsin, and acid; relative heat-lability; and partial stabilization to molar magnesium chloride. The virus was stable under freezing and thawing, and sonication. It was distinguished from avian encephalomyelitis virus by the neutralization test.