The electron microscope (EM) has been a mainstay for study of fastidious gastroenteritis viruses. Although 2nd and 3rd generation tests have been developed for the detection of "Norwalk" viruses and rotaviruses, EM remains an essential tool: (1) as the "supreme court" when newer tests yield variable results, (2) in the quest for new agents of viral gastroenteritis, (3) for visualizing the site of attachment of antibody on the virion in antigen-antibody reactions, (4) for serologic studies, (5) for direct visualization of virus particles, (6) for studying appropriate specimens derived from individuals with diseases of unknown etiology such as acquired immune deficiency syndrome (AIDS) and non-A, non-B hepatitis. The site of attachment of monoclonal antibody to the gene 8 product of rhesus rotavirus (serotype 3) was visualized to be on the outer capsid of the double shelled rotavirus particle. In addition, with such ascitic monoclonal antibody, several rotaviruses belonging to the third human rotavirus serotype could be easily serotyped by IEM. The site of attachment of monoclonal antibody to the gene 6 product of rhesus rotavirus was visualized to be on particles lacking the outer capsid (rough) but not on particles with an intact outer capsid (smooth). 27 nm particles were visualized in stools obtained from a large outbreak of gastroenteritis which was traced to contaminated cake frosting. Finally, about 50% of the episodes of pediatric diarrhea are still without any known etiology. A major effort should be made to examine by IEM or indirect IEM such "negative" specimens in an attempt to detect new etiologic agents.