During the 3 years from January 1996 to December 1998, a total of 33 strains of influenza C virus were isolated from 10,726 throat swab specimens collected from children with acute respiratory illness who visited two pediatric clinics in Yamagata City, Japan. These 33 strains were isolated in clusters during two different periods, 20 strains in May to August 1996 and the remaining 13 in March to June 1998. Antigenic analysis with monoclonal antibodies to the hemagglutinin-esterase (HE) glycoprotein and phylogenetic analysis of seven RNA segments showed that the 33 influenza C viruses isolated were antigenically and genetically similar and that they were reassortant viruses which had obtained PB2, PB1, HE, M, and NS genes from a C/pig/Beijing/115/81-like virus and P3 and NP genes from a C/Mississippi/80-like virus. These observations suggest strongly that during the survey period of 3 years, two outbreaks of influenza C occurred in Yamagata City, both of which were caused by a reassortant virus having the genome composition described above.