Attached or fallen branches of oak ( Quercus robur) colonized by Phlebia radiata and P. rufa were collected from locations in South-West Britain, and the patterns of decay within the branches analysed. Isolates were cultured from decayed wood, fruit-body tissue and from single basidiospores. Isolates were paired on malt agar in various combinations and also against other species of decay fungi. Bipolar incompatibility was found in both species, although P. rufa behaved inconsistently, and intraspecific pairings between genetically different dikaryons were mutually antagonistic. Pairings between the two species always resulted in strong mutual antagonism, immediately after hyphal fusion, and no dikaryons with clamp connexions were formed between homokaryons. Both species tended to replace other decay fungi in culture, but P. radiata was generally the more successful. All of the above interactions appeared to depend on contact with septate mycelia. Colonies of both species on agar extended relatively rapidly from a wide peripheral zone of appresed, mostly coenocytic mycelium which generally lacked hyphal fusions. Behind this zone, which was wider in P. radiata than in P. rufa, hyphae with clamp connexions developed and aerial mycelium was formed. The aerial mycelium of P. rufa was generally rather localized and rapidly aggregated into small hymenial surfaces on homokaryotic and dikaryotic mycelia. P. radiata formed a much more diffuse, floccose aerial mycelium which formed hymenia only occasionally on old cultures. Homokaryotic and dikaryotic cultures of both species produced oidia, although P. rufa did so more abundantly and more consistently than P. radiata. The cultural differences between the two species were correlated with differences in their ecology: P. radiata is regarded as a secondary colonizer, but P. rufa is probably a pioneer colonizing weakened, but still-living wood.