A somatic incompatibility system was demonstrated in Rosellinia necatrix (anamorph Dematophora necatrix) through barrage formation in pairings among mass fungal isolates from infected avocado trees in commercial orchards in southern Spain, as well as among single ascospore isolates derived from perithecia formed in infected roots. Results revealed the occurrence of high diversity in somatic incompatibility in the R. necatrix population in the study. The occurrence of R. necatrix perithecia in naturally infected avocado roots in the field suggests that such a diversity might originate from sexual reproduction. However, the possibility that other parasexual mechanisms demonstrated in this fungus may contribute to the genetic diversity can not be discounted.