Colorado has experienced high levels of Russian wheat aphid,Diuraphis noxia(Kurdjumov), damage and is considered an important site for release of biocontrol agents. In June 1990, field surveys were initiated to provide baseline data on the identity and prevalence of existing aphid pathogens, parasites, and predators. Sampling was conducted in spring-planted grain in irrigated and dry-land fields near Fort Collins and Akron.D. noxiawas the most abundant aphid; populations were high in both irrigated and dry-land fields. Parasite prevalence was low<5%) in all fields.D. noxiawas the most frequently parasitized aphid, and the most common parasite wasDiaeretiella rapae(M'Intosh). Syrphid fly larvae were the only predators consistently found feeding on aphids within theD. noxia–damaged (rolled) leaves. However, syrphid populations were low,<0.3 larvae per aphid-infested tiller. The ineffectiveness of the existing complex of parasites and predators in suppressingD. noxiapopulations underscores the need for introductions of more efficient biocontrol agents. Pathogenic fungi were active in nearly all fields; three species were found. In irrigated fields,Entomophthora chromaphidisBurger&Swain was the most common pathogen during the spring, with peak prevalence of 13% on 22 June.Pandora(=Erynia)neoaphidis(Remaudière&Hennebert) was not detected until late June, but it rapidly reached epizootic levels (44% infection by 18 July). Highest prevalence (20%) ofConidiobolus obscurus(Hall&Dunn) coincided with that ofP. neoaphidis. Prevalence of fungal pathogens in dry-land fields did not exceed 2.5%. The observation of only low levels of entomophthoralean fungal infection in the nonirrigated fields suggests that reliable use of these natural control agents against aphids in the semiarid West will require manipulation of environmental conditions through irrigation.