Function of rotavirus NSP1 was analyzed by using single-NSP1 gene-substitution reassortants, SKF, SDF, and SNF which have the NSP1 gene derived from human rotaviruses KU, DS-1, and canine rotavirus K9, respectively, in the genetic background of simian rotavirus SA11. The NSP1 genes from KU, DS-1, K9, and SA11 exhibited 58-76% nucleotide sequence identity to one another. No substantial difference in viral growth was observed among the reassortants and SA11. However, production of NSP1 was not detected in SNF when viral proteins were labelled with 35S-methionine during replication in MA104 cells, in contrast to SA11, SKF and SDF which exhibited evident expression of NSP1. Difference in reassortant formation was examined among the reassortant clones generated between human rotavirus strain 69M and either of SA11, SKF or SNF. Although reassortant formation rate was significantly lower in the cross 69M x SNF than the other crosses, selection rates of RNA segments from parent strain 69M in the resultant reassortants was similar among the crosses. Selectivity of homologous and heterologous NSP1 genes in SA11 background was also analyzed by mixed infection and multiple passages among the single-NSP1 gene-reassortants and/or SA11, KU NSP1 gene was selected most frequently, whereas homologous (SA11) NSP1 gene was least efficiently segregated. These results indicated that viral growth and genome segment reassortment with other viruses may not be influenced by the presence of heterologous NSP1 and its expression level, while genomic diversity of NSP1 genes might have been associated with the relative adaptability to the genetic background of SA11.