The genetic differences underlying Rhizoctonia solani populations provide a useful means for examining the nature and spread of the population within the rice system. So far, no attempt has been made to define variability in relation to spatial distribution and no information is available on the amount of variability in R. solani within the field. Many anastomosis groups are subdivided on the basis of the cultural, virulence, molecular, biochemical, immunological, and other characteristics into intraspecific groups (ISGs) (Ogoshi 1987). The most convincing validation of AG and ISG concepts has come from molecular systematic studies (Vilgalys and Cubeta 1994). Our study was undertaken to (1) analyze the interfield variability within 46 Indian rice isolates of R. solanicollected from two fields, one each at Seola-Kala, Dehradun District (hilly region, 24 isolates), and Nagina, Bijnore District (plain region, 22 isolates), for cultural and morphological characteristics, aggressiveness, anastomosis behavior, nuclear staining, and molecular characterization by RAPD analysis; (2) assess the agreement among five methods in differentiating the isolates; and 3) study the extent and possible factors responsible for intrafield variability in rice. This is the first attempt to define intrafield variability among R. solani isolates that cause sheath blight in rice.