ABSTRACTA unifactorial pattern of sexuality was documented for 13 stocks ofPhellinus torulosus.Incompatibility was indicated by a line of demarcation between the paired isolates; compatible pairings resulted in the formation of heterokaryotic secondary mycelium in the contact zone. Buller's phenomenon was not observed between paired heterokaryons and homokaryons. Both heterokaryons and homokaryons are without clamps and with binucleate hyphal segments. They can be distinguished as follows: 1) Fruiting only appears in heterokaryotic cultures; homokaryons never fruit. 2) Pairing of heterokaryons with other mycelia always results in the formation of a line of demarcation. 3) Nuclei with a DNA content of In, 2n and 4n can be found in heterokaryons; only In and 2n nuclei in homokaryons. 4) Growth of mycelium is slightly faster in heterokaryons as compared to homokaryons. Two partially intersterile groups ofP. torulosusare documented by pairing tests and ontogeny of secondary mycelia. One group consists of collections from Europe, mainly from the Mediterranean. The second group is restricted to the Canary Islands. No significant differences between the groups were noted in any cytological character or any macro- or micromorphological character examined. Fruiting was distinctly more common and faster in secondary mycelia derived from intragroup pairings as compared to secondary mycelia from intergroup pairings. No obvious substrate preferences were noted forP. torulosus.The taxon apparently occurs both as a parasite and a saprobe.