In studying the causes of human gastroenteritis, electron microscopy and related techniques have led to the identification of new viral agents that had previously escaped detection by routine cell culture procedures. Since the early 1970's, two major groups of gastroenteritis viruses have been identified by electron microscopy and immune electron microscopy: the Norwalk virus and rotavirus. This article addresses the detection of these two virus types and other viruses associated with gastroenteritis. Outbreaks of waterborne viral gastroenteritis associated with the Norwalk virus in Washington, Pennsylvania, and Georgia are described. Waterborne outbreaks in Sweden, Brazil, Colorado, the Soviet Union, East Germany, China, and Israel associated with rotavirus are discussed. The identification of the Snow Mountain Agent virus detected during an investigation of a gastroenteritis outbreak in Colorado is described. Includes 49 references, tables, figure.