Sodium ion is required for transport ofL-alanine and α-aminoisobutyric acid (AIB) into cells and membrane vesicles of a marine pseudomonad. Initial rate data obtained at various Na+and amino acid concentrations were tested for fit by least squares analysis to sequential and Ping-Pong equations. The sequential case is preferred statistically at the 99% confidence limit. Cotransport of AIB and Na+in cells could not be detected, perhaps explained by efflux of Na+shown to occur from these cells. The kinetic analysis is consistent with formation of a ternary complex involving Na+and the amino acid.