Japanese apricot ( Prunus mume) exhibits the S-RNase-based gametophytic self-incompatibility system as do other self-incompatible Prunus species. This report identifies the S haplotype-specific F-box protein gene ( SFB), a candidate gene for pollen- S, of Japanese apricot, which leads to the development of a molecular typing system for S-haplotype in this fruit species. Both 5'- and 3'-RACE (rapid amplification of cDNA ends) were performed with SFB gene-specific oligonucleotide primers to clone Pm-SFB(1) and Pm-SFB(7) of 'Nanko ( S(1) S(7))'. As in the case of SFB of other Prunus species, Pm-SFB(1) and Pm-SFB(7) showed a high level of S-haplotype-specific sequence polymorphism and their expression was specific to pollen. Genomic DNA-blot analyses of 11 Japanese apricot cultivars with the Pm-SFB probes under low stringency conditions yielded RFLP bands specific to the S(1)- to S(8)-haplotypes as well as a self-compatible S(f)-haplotype. A practical usage of SFB as a molecular marker for S-haplotypes and self-compatibility in Japanese apricot is discussed.