ABSTRACTDiploid-haploid pairings to identify field isolates ofArmillariato species generally rely on morphological differences between diploid and haploid colonies (flat and flufly colonies, respectively) and morphological changes in haploid tester colonies. Presumably, the change in morphology of the haploid tester is due to diploidization. Isozyme markers, aconitase and glucosephosphate isomerase, were used to follow nuclear migration from putatively diploid isolates into a haploid tester strain ofA. ostoyae.After pairing diploid field isolates with a haploid tester for 3 or 4 weeks, subcultures were taken from the hap!oid tester at 2 and 10 mm from the confrontation zone. Subcultures from 2 mm were mostly flat; subcultures from 10 mm were flufly, flat, or flufly with flat sectors. Hyphal tip cultures from the subcultures were analyzed for the isozyme markers. All flufly hyphal tip cultures had the isozyme phenotype of the haploid tester. Flat hyphal tip cultures either had the isozyme phenotype of the diploid parent or had a hybrid isozyme phenotype with markers of both the diploid parent and the haploid tester. The data suggest that in conspecific diploid-haploid pairings, the diploid nucleus usually migrates into the haploid mycelium and eventually displaces the haploid nucleus.