The RNA of strains of rotavirus obtained from patients hospitalized with diarrhea during two winter epidemics of rotaviral infection in successive years (November 1981 through April 1982 and December 1982 through April 1983) was analyzed by polyacrylamide gel electrophoresis. A single dominant electropherotype was found during the first two or three months of each epidemic. In contrast, various electropherotypes were identified during the latter portion of each epidemic. RNA patterns of the rotaviral strains that were dominant during the early phase of the two epidemics were different from each other.