This study unexpectedly detected a deletion in the promoter region of ras gene in two Korean strains of oak mushrooms, Lentinula edodes (Berk.). Sequencing of the promoter regions revealed that one type consisting of two strains had a 113 bp deletion in the region. The ras promoter region of Korean strains differed by 16 bases from that of the Japanese strains. Between the two types of Korean strains, except for the deleted portion, only a single site appeared to be different.