ABSTRACTWe examined nuclear division of germinating aeciospores of the western gall rust fungus,Peridermium harknessii(teleomorph,Endocronartium harknessii). Mature aeciospores were primarily (86%) binucleate. Migration of the two nuclei into the germ tube followed emergence of the germ tube. The percentage of germlings with 3 nuclei increased from 5% at 4 h to 32% at 12 h; thus, nuclear division in the germ tube was asynchronous and mitotic. We conclude that the fungus is an imperfect rust, and not an endocyclic rust, and should be calledPeridermium harknessii.