AbstractScanning electron microscopy was used to aid the study of the physical relationships between n+n stage ofCronartium ribicolaand its alternate host,Ribes hudsonianumvar.petiolare, and to examine the morphology of certain fungus structures. Germ tubes penetratedRibesleaves through stomates exclusively. Formation of appressoria over stomates were also observed. Intercellular leaf spaces were more than adequate to accommodate the intercellular mycelium. The intercellular hyphae and germ tubes were attached to adjacent host cell surfaces. Haustoria were found to be consistently saccate and shown to characterize the n+n stage ofC. ribicola.