The contribution of naturally occurring entomopathogenic fungi to regulation of aphid pests on agricultural crops has excited little enthusiasm on the part of crop protection specialists. Most who have written on the subject appear to have concluded that entomopathogenic fungi are usually of minor importance in the population dynamics of aphids. At low aphid densities, infection incidence is characteristically low; even so, the regulating effect of entomophthoran fungi on populations can be important. The nonfungicidal control had fewer aphids than any fungicidal treatment except benomyl on all sampling dates. The entomophthoran fungi causing mycoses in green peach aphid in 1985 were not identified to species. Outbreaks of many agriculturally important aphid species often end in sudden drastic collapse caused by epizootics of entomophthoran fungi. Concentrations of fungicide equivalent to the upper rates recommended for field use were generally toxic to green peach aphid in our laboratory tests.