Twenty four stool rotaviruses that comprised 22 distinct electropherotypes were selected for genome analysis from the collection of diarrheal specimens obtained over an eight-year period. These 22 electropherotypes were found in 46% of the total electropherotypes identified during the previous studies and represented 328 (64%) of rotavirus specimens in the collection. When genomic RNAs from these stool rotaviruses were hybridized to the 32P-labeled transcription probes prepared from prototypes representing three human rotavirus genogroups, Wa, DS-1, and AU-1, any one of the isolates showed a high degree of homology only with one of the three probes, which data confirmed and extended our previous observation on the existence of three distinct genogroups among human rotaviruses. Two stool rotaviruses which had an unusual combination of serotype (G1), subgroup (I) and RNA pattern (an identical short electropherotype), however, yielded the hybridization pattern indicative of an intergenogroupic single VP7 gene substitution reassortant. When they were cell culture adapted and analyzed by RNA-RNA hybridization, molecular evidence was obtained indicating that their VP7 gene derived from viruses belonging to the Wa genogroup whereas the remaining 10 genes hybridized with viruses belonging to the DS-1 genogroup. Interestingly, these natural reassortants emerged in the midst of the rotavirus season in which G1 strains predominated.