A concentrative uptake of arginine into brush border membrane vesicles (BBMV) from the midgut of Manduca sexta larvae was driven by an inwardly directed K+ gradient. The pH-dependence of the initial rate of arginine uptake between pH 7 and 10.5 paralleled the titration curve of the amino acid, suggesting that cationic arginine is the principal ionic form that is transported. In the presence of K+, at pH 7.4, arginine uptake was cis-inhibited and trans-stimulated by arginine and lysine but not by any other naturally occurring amino acids; it was also cis-inhibited by homoarginine and ornithine. Taken together, these data argue that arginine, lysine and their analogues share a cationic amino acid:K+ symporter (cotransporter), which we will designate as System R+. This novel symporter has a substrate spectrum similar to that of the uniporter, System y+, in that it accepts arginine+, lysine+, homoarginine+ and ornithine+ and rejects histidine. However, it differs from y+ in that it is cation-dependent and is almost inactive at pH 5.5.