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We present a colony-growth model for the red imported fire ant (Solenopsis invicta), compare predicted colony dynamics to field data, and use the model to examine the potential range of S. invicta in the United States. The model incorporates temperature-driven development rates of S. invicta life-stages to simulate the number of workers in a colony. Even though driving factors such as precipitation and competition were not included in the model, it tracks seasonal dynamics reasonably well, though the number of workers in the early stages of colony development is overestimated. The overwintering ability of the simulated S. invicta colony supports the −17.8°C January isotherm as the northern limit for range expansion. Simulations suggest that the ratio of minor to major workers is an important determinant of overwintering ability and S. invicta colony dynamics. The model suggests that other factors, such as alate …