Abstract: Cytology of mycelia, aeciospores, and aeciospore germ tubes of Korean pine (Pinus koraiensis) blister rust, Cronartium ribicola J. C. Fisch., were studied by the phase contrast microscope. The rust mycelia in the cortical parenchyma of the pine host bark, examined in paraffin sections, had one nucleus per cell. Mature aeciospores were usually binucleate. Upon germination, spores produced non-septate branched or unbranched germ tubes. Germinating spores usually produced one germ tube. No nuclear divisions occurred, and the two nuclei migrated together towards the tip of the germ tube. No appressorium and infection peg were observed at the tip of the germ tube. This type of nuclear behavior and germ tube growth is characteristic of host-alternating races, and it is distinguished from pine-to-pine races. Finally, the nature of heteroecious phenomenon of the Korean pine blister rust is also briefly discussed.