Many of the small round human fecal viruses implicated in outbreaks of nonbacterial gastroenteritis have been collected together and examined under the electron microscope. Negatively stained preparations without the addition of antibody were used so that the surface morphology of the virus particles remained clearly visible. It was apparent that several viruses, previously thought to be simply antigenic variants within the Norwalk group of viruses, show distinct morphological differences and quite clearly belong to other virus groups. By comparing the features of all the viruses examined in this study, both with each other and with standard cell culture strains of enterovirus, parvovirus, and calicivirus, it has been possible to propose an interim classification scheme, based primarily on the morphological appearance of the particles and supported by estimations of size and buoyant density.