Data on the natural control of cereal aphids,Diuraphis noxia(Mordvilko),Metopolophium dirhodum(Walker), andSitobion avenae(F.), by entomophthoralean fungi and hymenopterous parasitoids on furrow-irrigated spring wheat in southwestern Idaho are presented. The important fungi involved werePandora neoaphidis(Remaudire&Hennebert) Humber andConidiobolusspp., including C.obscurus(Hall and Dunn) Remaudire,C. thromboidesDrechsler, andC. coronatus(Constantin) Batko. During 1986–1989,M. dirhodumandS. avenaepopulations usually reached or exceeded economic levels.D. noxiadid not enter the region until late June 1987. It was the first aphid found infesting the crop in 1988 and reached high densities that summer but was not found in 1989, perhaps because of mortality during an extremely cold winter. Fungal infection occurred ≍2,3, and 6 wk after colonization of the crop byM. dirhodum, S. avenae, andD. noxia,respectively. Epizootics occurred each summer but usually after the crop was damaged by large aphid populations. An exception occurred during 1987 whenM. dirhodumandS. avenaepopulations were effectively suppressed by mycoses, apparently enhanced by unusually frequent rainfall during late May and June.M. dirhodumexperienced much higher mortality from fungal infection than didS. avenaeandD. noxia. P. neoaphidiswas the dominant fungus infectingM. dirhodumandD. noxia,whereasConidiobolusspp. were most important onS. avenae. Entomophthora chromaphidisBurger&Swain and twoZoophthoraspecies sporadically infected cereal aphids. Parasitoids, mainlyAphidius erviHaliday forS. avenaeandM. dirhodumandDiaeretiella rapae(M'Intosh) forD. noxia,usually attacked cereal aphids earlier than the fungi but were less influential during the decline of host populations. Multiple regression and correlation analysis indicated that mycoses and parasitoids made significant contributions to the reduction of peak populations of each aphid species compared with the influence of crop maturation. Weak correlation between fungal infection and host density and climatic factors (r2ranging from 0.35 to 0.53) was found. Development of fungal infection seemed to be better correlated with host density than with climatic factors. Precipitation was more associated with fungal infection ofS. avenae,which inhabits upper portions of the host crop, but had little effect on that ofM. dirhodumandD. noxia,which inhabit more humid microenvironments. No other climatic factors were significantly correlated with fungal infection. A hypothesis is proposed that the site on a wheat plant typically occupied by an aphid species influences the development of mycoses.