The fungal pathogen, 'Cryphonectria parasitica', introduced to North America in the early 20th century, has reduced natural populations of 'Castanea dentata' (American chestnut) by 99 percent. The remaining individuals are often interspersed with non-native ' Castanea' species; however, the existence of hybrids has not been tested. My research characterizes four putative parental species morphologically and genetically (randomly amplified polymorphic DNA), and examines the remnant ' Castanea' populations of southern Ontario for hybrids. Both methods were able to distinguish between 'C. dentata' and three non-native ' Castanea' species, but only the molecular techniques could clearly identify nonnative and hybrid 'Castanea' trees in Ontario forests. One hybrid of American and Japanese chestnut species was detected in an Ontario woodlot, showing for the first time that non-native genetic material is entering the Canadian 'C. dentata' population. Canadian chestnut is also somewhat distinct from American chestnut in morphology and genetics.