Laboratory studies demonstrated that temperatures down to −15°C had little effect on mortality of adult viviparous, parthenogeneticMyzus persicae(Sulzer) collected from overwintered spinach,Spinacia oleraceaL. The LT50and LT95values at −15°C were 68.61 and 87.07 h, respectively. Aphid survival at −17°C, however, was significantly reduced. The LT50and LT95values were 7.53 and 9.31 h, respectively. Additional temperature reductions in laboratory tests reduced aphid survival further. Field samples collected from overwintered spinach in 1985 revealed significant reductions in aphid abundance and survival following extremely low temperatures (−20.6 and −21.7°C). Furthermore, aphid survival was significantly greater on sheltered plant locations. Significant reductions inMyzus persicaeabundance and survival were not detected during the mild winter of 1986 when the lowest temperature was −12.8°C. At the 1986 harvest, the mean number of live aphids per plant was 37-fold greater than in 1985.