Calicivirus was detected in 8 (1.2%) of 647 hospitalized patients during a survey of acute gastroenteritis in infants and young children, conducted between December 1974 and September 1977. Morphologically calicivirus was approximately 30 nm in diameter with an easily recognizable staining "star of David" configuration. Its buoyant density in cesium chloride was 1.38-1.40 gm/ml. The serologic response to calicivirus by immune electron microscopy (IEM) was demonstrated only in paired sera from patients who shed the virus in their stools. The results suggest that calicivirus might be a cause of acute gastroenteritis in infants and young children.