The nucleotide sequences of a 300 bp segment carrying the small hydrophobic (SH) protein gene of a large number of virus strains, belonging to 10 different mumps virus genotypes denoted A-J, were compared. When virus strains belonging to the same genotype were compared intra-genotypically, a variation in the range 2-4% was recorded. The level of inter-genotypical variation was higher (8-18%). A consistent finding was the more pronounced variation found when the A genotype was compared to the B-J genotypes than when the B-J genotypes were compared among themselves. A similar relatively more pronounced genetic variation between the A and B-D genotypes has also been found previously with the hemagglutinin-neuraminidase and fusion protein genes. It is concluded that the establishment of a new genotype should be founded on a nucleotide variation of the SH gene of > or = 6% in relation to previously described genotypes.