Seed coating treatments of sunflower by the systemic insecticide imidacloprid was suspected of affecting honey bees and bumblebees. The hypothesis raised was whether imidacloprid could migrate into nectar and pollen, then modify flower attractiveness, homing behavior, and colony development. Our greenhouse and field experiments with Bombus terrestris L. were aimed at the following: the behavior of workers foraging on treated and control plants blooming in a greenhouse, the homing rate of colonies placed for 9 d in a treated field compared with colonies in a control field, and the development of these 20 colonies under laboratory conditions when removed from the fields. In the greenhouse, workers visited blooming heads of treated and control plants at the same rate and the mean duration of their visits was similar. In field colonies, analysis of pollen in hairs and pellets of workers showed that in both fields 98% of nectar foragers visited exclusively sunflowers, whereas only 25% of pollen gatherers collected sunflower pollen. After 9 d, in the control and treated field, 23 and 33% of the marked foragers, respectively, did not return to hives. In both fields, workers significantly drifted from the center to the sides of colony rows. During the 26-d period under field and laboratory conditions, the population increase rate of the 20 colonies was 3.3 and 3.0 workers/d in hives of the control and treated field, respectively. This difference was not significant. New queens were produced in eight colonies in either field. The mean number of new queens per hive was 17 and 24 in the control and treated field, respectively. Their mating rate was the same. It was concluded that applying imidacloprid at the registered dose, as a seed coating of sunflowers cultivated in greenhouse or in field, did not significantly affect the foraging and homing behavior of B. terestris and its colony development.