High affinity binding sites for serine and aspartate have been characterized in membranes from Salmonella typhimurium and Escherichia coli. Greater than 80% of these sites have been identified as chemotaxis receptors. Mutants lacking binding sites for these amino acids have been shown to have corresponding defects in taxis. The substrate specificity of each of the receptors in Salmonella is very high; most analogs of serine and aspartate do not bind to these receptor sites and do not affect chemotaxis. The transport of these amino acids is apparently not related to chemotaxis. At least 2500 serine receptors and 1200 aspartate receptors with dissociation constants of about 5 microM are present in the membrane fraction of logarithmically growing cells.