Studies were conducted over a one-year period at 6 locations throughout the range of the imported fire ant,Solenopsis saevissima richteriForel, to determine regional variations in the seasonal activity of this pest. Threshold levels for various types of activity were found to be uniformly temperature-dependent in all parts of the range. Workers began to forage when soil temperatures exceeded 10°C. Some foraging occurred even in the winter months in the northern part of its range when the daily temperature exceeded this threshold for more than a few hours. Brood production in spring began when the weekly mean soil temperature (at 5 cm) rose above 10°C, worker and sexual pupae appeared at ca. 20°C, and the alates at 22.5°C. In Florida the period of favorable soil temperature for new colony-founding and growth (24°C) lasted ca. 198 days; in northern Mississippi it lasted 83 days. Similarly, the ants in Florida were found to consume twice as much food annually and to produce 1.5 times more brood than the ants in the northern part of the range.