A study was undertaken to find out the extent of variability and the probable cause of its emergence in natural populations of the spot blotch pathogen Bipolaris sorokiniana of wheat. The pathogen was isolated from infected seeds and leaves on PDA and purified by monoconidial isolation. The frequency of nucleus in mycelial cells of isolates was recorded using fluorescent staining. Based on colony morphology, the isolates were grouped into five groups. Cluster analysis confirmed this classification. Maximum frequency (44.63 %) of isolates in the natural populations was that of black suppressed type. This group was most aggressive and identified as the epidemic population. Lowest frequency (4.96 %) was observed for white isolate which produced very few conidia. The mean radial growth of isolates on 8 th day ranged form 4.77 to 8.27 cm. The monoconidial isolates, when inoculated on resistant and susceptible cultivars, showed mean AUDPC range of 176.67-1237.59 and 600.83-2144.21 % days, respectively. In the epidemic population, cells with two nuclei displayed highest frequency, while in other groups, cells with three or four nuclei were most frequent. The present study revealed that the variability in Bipolaris sorokiniana was reflected in different combinations of the nuclei.