During two nosocomial outbreaks in a tertiary care nursery, 22 of 102 infants had infections due to human rotavirus (HRV). The time sequence and proximity of the infected infants in the nursery rooms had suggested infant-to-infant spread of a single HRV strain in each outbreak. Polyacrylamide gel electrophoresis of virus permitted demonstration of four different HRV electropherotypes (strains) during the first outbreak period and seven during the second. One of the strains was detected during both outbreaks. All strains had fast-migrating RNA segments 10 and 11. Infant-to-infant spread of at least two different viruses apparently occurred in each outbreak period. Thus, electrophoretic analysis showed that infants in a tertiary care center can be infected with at least 10 different HRV strains within two months, that multiple HRV strains can spread independently and in parallel to different infants as part of a "single" outbreak, and that secondary spread of individual strains can be traced with considerable accuracy.