We conducted a series of field experiments to assess whether cool-season grasses were important in maintaining populations of the Russian wheat aphid, Diuraphisnoxia (Mordvilko), between the summer harvest and fall sowing of winter cereals. All six grass species in a common garden in northern Utah supported aphid population growth from May to September 1991, although aphid densities were unusually low on mature foliage in July. Under conditions of little or no plant competition, each grass species produced at least a few fall tillers before mature shoots had senesced completely and hence provided a continuous source of food for aphids. Indian ricegrass was a consistently goodhost and bluebunch wheatgrass was a consistently poor one, but the relative suitabilities of the six hosts varied considerably over the season. Neither plant water content nor total nitrogen was correlated with differences in aphid performance among hosts. We conducted a greenhouse experiment to compare estimates of plant quality under field and greenhouse conditions. Differences in aphid perfornlance among hosts in the greenhouse resembledthose observed in the field early in the season, but not during the critical “oversummering” period.