To address the question whether there was any molecular evidence for interspecies transmission of rotaviruses from one animal species to another, genetic relationships among human and animal rotaviruses were examined by a series of hybridization experiments in which genomic RNAs from 14 rotavirus strains derived from seven different host species were hybridized with the [32P]-labelled transcription probes prepared from 11 strains representing rotaviruses from those seven host species. In general, higher level of homology among most, if not all, of the cognate gene segments that allowed classification into the same genogroup was shared among rotaviruses recovered from the same animal species but this level of homology was not found among rotavirus strains derived from different host species. However, such a high level of homology that was usually found among rotaviruses recovered from the same animal species was detected between feline rotavirus strain Cat97 and canine rotavirus strain K9 as well as between human rotavirus strain AU-1 and feline rotavirus strain FRV-1. The sharing of closely related genetic constellation of most of the 11 gene segments (genogroup) by rotaviruses recovered from different animal species provided molecular evidence that interspecies transmission of rotaviruses occurred in nature at least recently in the evolutionary history.