The surface structure of the echinulate urediniospores of Puccinia coronata f. sp. avenae race 320 was examined with the light microscope and the electron microscope. The scanning electron microscope revealed a network of ridges encircling the spines. On many urediniospores, small ring-like ridges, about 1 μ across, closely encircled the bases of the spines and sometimes were connected by fine linear ridges. On other spores, the ridges took the shape of four- to seven-sided polygons, 2–3 μ across, with triangles at their corners.