The S1 gene nucleotide sequences of 10 type 3 (T3) reovirus strains were determined and compared with the T3 prototype Dearing strain in order to study sequence diversity in strains of a single reovirus serotype and to learn more about structure-function relationships of the two S1 translation products, sigma 1 and sigma 1s. Analysis of phylogenetic trees constructed from variation in the sigma 1-encoding S1 nucleotide sequences indicated that there is no pattern of S1 gene relatedness in these strains based on host species, geographic site, or date of isolation. This suggests that reovirus strains are transmitted rapidly between host species and that T3 strains with markedly different S1 sequences circulate simultaneously. Comparison of the deduced sigma 1 amino acid sequences of the 11 T3 strains was notable for the identification of conserved and variable regions of sequence that correlate with the proposed domain organization of sigma 1 (M.L. Nibert, T.S. Dermody, and B. N. Fields, J. Virol. 64:2976-2989, 1990). Repeat patterns of apolar residues thought to be important for sigma 1 structure were conserved in all strains examined. The deduced sigma 1s amino acid sequences of the strains were more heterogeneous than the sigma 1 sequences; however, a cluster of basic residues near the amino terminus of sigma 1s was conserved. This analysis has allowed us to investigate molecular epidemiology of T3 reovirus strains and to identify conserved and variable sequence motifs in the S1 translation products, sigma 1 or sigma 1s.