DNA base sequence homology among isolates within each anastomosis group of Rhizoctonia solani, AG-3, AG-5, AG-7 and AG-BI, was investigated on the basis of DNA-DNA reassociation kinetics to examine their genetic relatedness. DNA homology value of isolates within each anastomosis group ranged from 97.6 to 100% with AG-3 (potato type, IV), from 94.0 to 98.2% with AG-5, from 95.6 to 98.8% with AG-7, and from 91.4 to 100.2% with AG-BI. These results suggested that each of these anastomosis groups is genetically homogenous.