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AbstractThe soilborne plant pathogenic basidiomycetous fungus Thanatephorus cucumeris(anamorph: Rhizoctonia solani) is composed of at least 12 anastomosis groups (AG), which seem to be genetically isolated. The sexuality of T. cucumerishas been studied for several AG, revealing that AG 1 and 4 are heterokaryotic and possess a bipolar mating system, and that AG-4 is heterothallic. To clarify whether AG-1 is also heterothallic, fruiting of heterokaryotic and homokaryotic single spore-derived isolates was studied and nuclear staining with fluorochromes was performed during fruiting. Meiosis preceding spore formation was studied for heterokaryotic and derived homokaryotic isolates. A few homokaryotic isolates did not form fruiting bodies. Most homokaryotic isolates formed basidia, but showed interrupted meiosis. A few homokaryotic isolates completed meiosis and formed viable basidiospores. The observed meiotic homokaryotic fruiting in T. cucumerisAG-1-IC differed from the mitotic homokaryotic fruiting of most other basidiomycetes. More than six chromosomes were observed. Heterokaryons formed by pairing of two nonfruiting homokaryons were able to fruit, and the amount of spores discharged by fruiting heterokaryons was 500 times higher than for fruiting homokaryons. From this it was concluded that T. cucumerisAG-1-IC is heterothallic.