ABSTRACTHomokaryon-homokaryon and heterokaryon-homokaryon interactions of sibling strains of the ectomycorrhizal fungusPisolithus arhizuswere investigated. Pairings between sexually compatible homokaryons gave rise, almost always, to heterokaryotic mycelium in both of the progenitors, but in some cases heterokaryotic mycelium was present in only one of the progenitors. This unidirectional movement of nuclei and the speed of nuclear movement was followed in six representative pairings of homokaryons and complete heterokaryotization occurred rapidly, within 4 days, in one progenitor of one pairing. In heterokaryon-homokaryon pairings, heterokaryotization of the homokaryon was a consistent feature in all legitimate mating type pairings except in one case. Heterokaryotization did not occur in illegitimate pairings. The nuclear constitution of three analyzed heterokaryotized colonies was found to be a composite genotype containing one nucleus from the donor heterokaryon and one nucleus from the recipient homokaryon. This study is the first report of the homokaryon-homokaryon and homokaryon-heterokaryon nuclear interactions ofP. arhizus.