Field isolates of Rhizoctonia solani are presumed to consist of heterokaryotic cells, but the number of different nuclear types present in each cell is unknown, nor is it known if there is any regulation of the heterokaryotic state. To investigate the nature of heterokaryosis in AG 4, hyphal tip subcultures of two strains were examined using vegetative compatibility as a marker. Upon subculture, each isolate could be placed into one of two VCGs, members of which were compatible within the group but incompatible with those of the other group and the parent strain. No other phenotype could distinguish these groups from each other or the parents. Pairing of hyphal tip isolates never resulted in regeneration of the parental strain. Examination of nuclei in the tip cells of incompatible strains by DAPI staining and fluorescence microscopy showed variation in nuclear numbers in different parts of the fungal colony and a significant tendency for some adjacent branch tips to have a similar number of nuclei. These results suggest a mosaic of nuclear, and perhaps mitochondrial, types within the colony, and that subcultures of components of the mosaic retain their differences. This variation implies that regulation of nuclear number within a colony is loosely controlled.