Motile, but generally nonchemotactic (che) mutants of Escherichia coli were examined for complementation and recombination with specialized lambdafla transducing phages. The complex complementation behavior of these mutants found previously in F-prime tests could largely be accounted for by intragenic complementation and by polarity effects. Mutants of the "cheA" class defined two genes, cheA and cheW, which appeared to be contranscribed. Mutants of the "cheB" class defined four genes, cheX, cheB, cheY, and cheZ, which also constituted a transcriptional unit. Mutants defective in cheA, cheW, cheX, or cheY function swam smoothly, with little or no tumbling, whereas cheB or cheZ mutants exhibited very high tumbling rates. These functions are probably involved in initiating of controlling changes in flagellar rotation in response to chemotactic stimuli.