Uptake of tritiated leucine, alanine, and phenylalanine was measured at the physiological pH of 10 by rapid filtration in brush-border membrane vesicles from the midgut of the larval tobacco hornworm, Manduca sexta. A 20-fold excess of unlabeled leucine, isoleucine, methionine, valine, alanine, lysine, histidine, phenylalanine, and glutamine inhibited uptake of leucine and phenylalanine, and six of these amino acids inhibited uptake of alanine, by more than 50% both in the presence and absence of a potassium ion gradient. These inhibitory amino acids also drove countertransport of leucine, alanine, and phenylalanine with accumulation ratios exceeding 2. These results are consistent with the hypothesis that leucine, alanine, and phenylalanine share a common uptake system - a broad scope B type symporter - which interacts strongly with half of the commonly occurring amino acids, interacts moderately with an additional quarter of them, but does not interact with cysteine, arginine, glutamate, aspartate, or proline.