Between December 1976 and January 1978, infection with rotavirus was detected by electron microscopy in 61 (25%) of 242 infants and young children hospitalized with acute gastroenteritis at two hospitals in Mexico City. This type of infection was more frequent in autumn than in winter. The presence of virus could also be detected by the electrophoretic pattern of its segmented, double-standed ribonucleic acid (RNA) in the stool samples taken from 52 (85%) of the 61 patients who were shown by electron microscopic examination to excrete rotavirus. Two distinct patterns, previously called 2s and 21, were observed, which were distinguishable by the relative migration of the second, 10th, and 11th viral RNA segments in gel electrophoresis; the 21 pattern was observed much more frequently than with the 2s pattern.