The toxicity of the metalloids arsenic and antimony is related to uptake, whereas detoxification requires efflux. In this report we show that uptake of the trivalent inorganic forms of arsenic and antimony into cells of Escherichia coli is facilitated by the aquaglyceroporin channel GlpF and that transport of Sb(III) is catalyzed by the ArsB carrier protein; everted membrane vesicles accumulated Sb(III) with energy supplied by NADH oxidation, reflecting efflux from intact cells. Dissipation of either the membrane potential or the pH gradient did not prevent Sb(III) uptake, whereas dissipation of both completely uncoupled the carrier protein, suggesting that transport is coupled to either the electrical or the chemical component of the electrochemical proton gradient. Reciprocally, Sb(III) transport via ArsB dissipated both the pH gradient and the membrane potential. These results strongly indicate that ArsB is an antiporter that catalyzes metalloid-proton exchange. Unexpectedly, As(III) inhibited ArsB-mediated Sb(III) uptake, whereas Sb(III) stimulated ArsB-mediated As(III) transport. We propose that the actual substrate of ArsB is a polymer of (AsO)(n), (SbO)(n), or a co-polymer of the two metalloids.