Russian wheat aphids, Diuraphis noxia (Mordvilko), were exposed to a killed, freeze-dried preparation of ice nucleating active bacteria (Pseudomonas syringae) by ingestion in sucrose solution, and topically by misting them with the bacteria in distilled water. Untreated (dry) Russian wheat aphids consistently supercooled to –25.0°C before internal ice nucleation occurred. Ingestion of P. syringae suspensions of 10–150 mg/ml in 10% sucrose elevated the supercooling point an average of 16.3°C compared with those fed only 10% sucrose. Topical application of R syringae in a water suspensions elevated the supercooling point an average of 9.0°C compared with aphids exposed to distilled water. Supercooling points from aphids that ingested P syringae were more consistent and higher than for aphids exposed topically. This result would be expected because the gut provides the bacteria direct contact with body water. The soil surface temperature in winter wheat (winter 1993–1994) compared with the supercooling temperatures in these experiments shows that the Russian wheat aphid would be significantly more susceptible to winter temperatures and lethal internal ice formation after being exposed to P. syringae. P. syringae has the potential for use as a microbial insecticide that would decrease winter survival of the Russian wheat aphid by reducing its supercooling capacity.