A 7-yr study on field plots in Lleida, northeastern Spain, was carried out to determine the factors that playa major role in the population dynamics of cereal aphids in winter under Mediterranean conditions. Plots were monitored at 7–10-d intervals from crop emergence until the first fortnight in April using a D-vac vacuum net set. Several weather and host plant parameters were related to the following aphid population changes: (1) seasonal net rates of aphid increase (from the first sampling in October or November to 31 March), (2) net rates of increase, and (3) k mortality values between consecutive samplings. Population dynamics ofRhopalosiphum padi(L.) andSitobion avenae(F.), the species found most frequently, were affected, not only by low temperatures causing aphid mortality but also hy other factors. Dry and cold weather, through the effect they have on host plant phenology and quality, reduce aphid developmental rates. Winter minimum temperatures and rainfall, the latter by its effect on host plants, seem to be the major weather parameters to consider to forecast aphid population levels at the beginning of spring.