To define more completely the period of fecal excretion of virus during hepatitis A virus infection, we studied 24 fecal samples from six children with clinical illness during an epidemic of type A hepatitis. As determined by immune electron microscopy, the six patients had detectable viral excretion before or by the time of the first abnormality in serum glutamic-pyruvic transaminase (alanine aminotransferase). Viral excretion reached a peak early and declined to undetectable levels before levels of serum enzyme reached a peak. These data accord with epidemiologic evidence that the person who already has symptoms and signs of type A hepatitis is unlikely to transmit the infection to others. Immune electron microscopy, therefore, may be a better index to the period of communicability than studies of experimental infection in human subjects. This conclusion would imply that precautions against fecal contamination are not usually necessary for patients hospitalized with type A hepatitis.