In this study, we attempted to establish a simple detection method for classification of IBV S1 genotypes by direct reverse transcriptase-polymerase chain reaction (RT-PCR). Then, to evaluate the usefulness of the S1 genotype-specific RT-PCR, we examined the relationship between S1 genotypes and serotypes of IBV in Japan. Sequencing of the S1 genes of IBV and phylogenetic tree analysis were conducted. On the basis of the sequencing data of the S1 genotype samples, we determined primer sets specific for each genotype. Five vaccine strains in Japan as reference strains and 46 field isolates were classified into different genetic clusters by phylogenetic tree analysis (JP-1, JP-II, JP-III, Mass and 4/91) and were matched to the results of S1 genotype-specific RT-PCR. A cross virus-neutralizing test showed that the five vaccine strains in Japan exhibited different serotypes from each other. The concordance rate of the 46 field isolates between the S1 genotypes and serotypes was 65.2%. The present study indicates that genotype-specific RT-PCR could be a convenient and useful tool for determining IBV serotypes and could contribute to the control of IBV outbreaks in Japan.