Fipronil is a potent insecticide used for control of termites, fleas, roaches, ants, and other pests. We measured fipronil, fipronil sulfide, and desulfinyl fipronil concentrations in indoor and outdoor dust from 24 residences in Austin, Texas. At least one of these three fipronil compounds was detected in every sample. Fipronil accounted for most of the total fipronil (T-fipronil; fipronil + desulfinyl fipronil + fipronil sulfide), followed by desulfinyl fipronil and fipronil sulfide. Nineteen of 24 samples of indoor dust had T-fipronil concentrations less than 270 microg/kg; the remaining five had concentrations from 1320 to 14,200 microg/kg. All three of the residences with a dog on which a flea-control product containing fipronil was used were among the five residences with elevated fipronil concentrations. In outdoor dust all concentrations of T-fipronil were less than 70 microg/kg with one exception (430 microg/kg). For every residence, the concentration of T-fipronil in indoor dust exceeded that in outdoor dust, and the median concentration of T-fipronil was 15 times higher indoors than outdoors.