The susceptibility to fipronil of U.S. and French populations of Reticulitermes flavipes (Kollar) (Isoptera: Rhinotermitidae) was evaluated in two types of laboratory bioassays: contact intoxication with 0.01–1 ppm treated sand and feeding intoxication with 0.1–10 ppm treated filter paper. Contact intoxication with 0.01 ppm fipronil caused 60% mortality after 55 and 64 h in the French and U.S. populations, respectively, whereas in the 5 ppm feeding assay 70 and 60% mortality was observed after 7 d in the French and U.S. populations, respectively. We evaluated the uptake, clearance, and transfer of fipronil among workers of French R. flavipes using [¹⁴C]fipronil in contact (0.01 ppm) and feeding (3 ppm) bioassays. Fipronil amounts were measured on their cuticle and in their bodies. Maximal uptake was observed in the contact assay. A significant uptake of fipronil occurred in the feeding assay. Transfer from exposed donors to unexposed recipients occurred within 24 h. Frequent horizontal transfer resulted in a significant uptake in recipients, particularly when donor fipronil acquisition was by feeding. Donors transferred ≈46% of the toxicant to recipients. Social behaviors such as contact and grooming, together with internalization of the biocide, may be components of the horizontal transfer process and contribute to the efficacy of fipronil in the field.