Generally nonchemotactic mutants of Escherichia coli and Salmonella typhimurium were analyzed by interspecies complementation tests to determine the functional correspondence between the che genes of these two organisms. The E. coli che region was introduced into Salmonella recipients by means of a series of F-prime elements. Wild-type che genes of E. coli F'420 complemented all che mutants of Salmonella except cheS, cheV, and a subclass of cheU. A series of tester episomes carrying E. coli che mutations were then used to determine which E. coli che function was responsible for the complementation of each Salmonella che defect. By this method, the following correspondences were determined: cheAE and chePS,cheWE, and cheWS, cheXE and cheRS, cheYE and cheQS, cheCE and cheUS, cheBE and cheXS, and cheZE and cheTS. (The subscripts E and S refer to E. coli and S. typhimurium genes, respectively.) In some tests, especially those involving the last two pairs of genes, poor complementation was observed between noncorresponding genes. A model explaining these observations in terms of subunit interactions is proposed.