ABSTRACTMating tests were performedin vitroto determine interfertility patterns among eastern North American populations ofPeniophora cinera, and two sympatric sibling species of theP. cinereacomplex in Europe. No intersterile groups were detected in eastern North America. Reproductive isolation between the two European species reported previously by Hallenberg was verified. Since one sibling species is apparently restricted to the wood ofFagus sylvatica, speciation accompanied by substratum specialization is indicated. Intercontinental pairings showed that AmericanP. cinereais partially reproductively isolated from the two European sibling species.