In the Pacific Northwest region of the United States, cool-season grasses grown for seed can be severely damaged by Puccinia graminis subsp. graminicola, causal agent of stem rust. Urediniospores of the pathogen, collected either from perennial ryegrass (Lolium perenne) or tall fescue (Festuca arundinacea), were tested for host range among selected grasses and cereals. Under greenhouse conditions, the inoculum from L. perenne could produce pustules on this host, as well as on Dactylis glomerata, Lolium multiflorum, Poa pratensis, and F. rubra subsp. rubra and subsp. commutata; it caused only limited pustule development (low incidence or pustule type) on F. arundinacea, F. ovina subsp. hirtula, P. annua, Hordeum vulgare, and Secale cereale. No symptoms were produced on Triticum aestivum or Avena sativa. The inoculum from F. arundinacea had a host range that included itself, D. glomerata, L. perenne, L. multiflorum, and F. rubra subsp. rubra and subsp. commutata; there was no sign of pustule development on Poa spp. or the cereal grains tested (T. aestivum, A. sativa, S. cereale, and H. vulgare). The two urediniospore populations differed also in rate of symptom development on most of their common hosts. There was a small, but statistically significant, difference in spore size among the populations from different hosts. No recommendation is made for separate taxonomic status of populations from F. arundinacea and L. perenne, but the adaptation of each to its own host should be considered when devising disease management strategies and studying host genetic resistance.