Approximately 50 single-basidiospore isolates (SBIs) obtained from each of 16 field isolates of Thanatephorus cucumeris AG-1 IC were examined for heterokaryon formation. All SBIs obtained from each field isolate were divided into two mating groups (SBIs-M1 and SBIs-M2), and tufts of mycelia were formed in the contact zone between colonies of paired SBIs-M1 and -M2 based on 0.5 % charcoal agar medium. Tufts were produced from all possible pairing between SBIs from non-parental field isolates. Hyphal anastomosis reactions indicated no cell death and random cell death at the contact cell, and was not related to tuft formation. AFLP phenotypes of SBIs from each field isolate were not identical to each other and were different from their parental field isolate. AFLP phenotypes of the tuft isolates formed from SBIs-M1 and SBIs-M2 from each field isolate were heterokaryotic. Moreover, several SBIs also formed tufts with their parental and non-parental field isolates. AFLP phenotypes of these tuft isolates suggested that they were all heterokaryotic. Results of these experiments suggest that T. cucumeris AG-1 IC is heterothallic and bipolar, and that genetic exchange can occur between homokaryotic and heterokaryotic isolates (Buller phenomenon).