Neutralizing serum antibody to serotype 9 rotavirus WI61 was detected in 41% of 870 Ecuadorian children and 26% of 140 German children. In both areas an age-related prevalence increase was observed. We identified 11 serum samples from Ecuadorian children which neutralized exclusively serotype 9 rotavirus. Thirteen of 71 (18%) German children hospitalized with serologically defined primary rotavirus gastroenteritis showed a seroconversion to serotype 9 rotavirus; however, in 10 of these 13 patients, the infecting serotype could be identified as serotype 1, 3, or 4. Furthermore, all 13 patients showed fourfold increases in titer to at least one further serotype.