Analysis of 213 field isolates of Botrytis cinerea and 240 ascospore isolates of its sexual form Botryotinia fuckeliana indicated that sexual compatibility of this fungus is controlled by a single mating type gene with two alleles. Most isolates were heterothallic, that is, they were self-sterile and able to produce ascospore progeny when crossed with reference strains carrying the mating type gene MAT1-1 or MAT1-2. About 16% of the field isolates and 6% of the ascospore progeny were homothallic, that is, self fertile and compatible with both MAT1-1 and MAT1-2 strains. Both mating types are widespread in nature. The close association of MAT1-1 and MAT1-2 field isolates on various hosts in several regions of Italy shows that sexual reproduction and meiotic recombination might be an important source of genetic variation in this pathogenic fungus.