Phellinus weirianus (Aphyllophorales: Hymenochaetaceae) causes a white heartrot of living Juglans and is the major cause of decay in Arizona black walnut. Infection is caused by germinating basidiospores. Tissue isolates fruit in culture, but single basidiospore isolates do not. The growth of tissue and single-spore isolates is slow to very slow. Cultural morphology is quite variable even among single-spore isolates derived from a single, culturally produced basidiocarp. Single-spore isolates, presumed to be homokaryotic, are derived from uninucleate spores that germinate and develop hyphae with paired nuclei. Lack of fruiting in homokaryons suggests that P. weirianus is heterothallic. Tissue isolates derived from basidiocarps produced in nature are assumed to be heterokaryotic. Hyphae of tissue isolates have nuclei that are not paired and are more numerous than those in single-spore isolates. Interaction zones are formed in pairings of heterokaryons, pairings of homokaryons, and pairings of homokaryons with the parent heterokaryon. Homozygous matings do not form an interaction zone. Agar-block decay tests show that single-spore isolates exhibit no appreciable differences in decay capacity from tissue isolates. Phellinus weirianus readily decays woods of associated riparian species (Arizona alder, velvet ash, southwestern chokecherry, netleaf oak, and Arizona sycamore) in standard wood test blocks although it is not known on these hosts in nature.