Canker stain of plane tree recently was reported in a small area of southwestern Greece on natural populations of the important riparian species, oriental plane tree, Platanus orientalis. The fungus Ceratocystis platani (= C. fimbriata f. platani) was successfully isolated from infected, stained wood of 15 dead or dying trees on the Peloponnese Peninsula. Genetic analyses of these 15 isolates from Greece, using nuclear and mitochondrial DNA fingerprints, showed the fungus to be identical to the genotype reported from Italy, France, and Switzerland. A polymerase chain reaction-based microsatellite analysis of eight polymorphic loci discovered a new microsatellite allele in one of the isolates from Greece, but this may be due to a mutation after introduction of a single strain. Earlier studies indicated that the most common European genotype had been introduced from eastern North America to Italy during World War II. The recent introduction to Greece appears to have originated from Italy, France, or Switzerland, rather than from eastern North America, where the fungus is native. The pathogen is having a dramatic impact on the natural population of P. orientalis in southwestern Greece, and containment measures should be imposed before it spreads throughout the natural range of this ecologically and historically important host.