The chemotaxis gene cluster from the photosynthetic bacterium Rhodospirillum centenum contains five open reading frames (ORFs) that have significant sequence homology to chemotaxis genes from other bacteria. To elucidate the functions of each ORF, we have made various mutations in the gene cluster and analyzed their phenotypic defects. Deletion of the entire che operon (delta che), as well as nonpolar disruptions of cheAY, cheW, and cheR, resulted in a smooth-swimming phenotype, whereas disruption of cheB resulted in a locked tumbly phenotype. Each of these mutants was defective in chemotactic response. Interestingly, disruption of cheY resulted in a slight increase in the frequency of tumbling/reversal with no obvious defects in chemotactic response. In contrast to observations with Escherichia coli and several other bacteria, we found that all of the che mutant cells were capable of differentiating into hyperflagellated swarmer cells when plated on a solid agar surface. When viewed microscopically, the smooth-swimming che mutants exhibited active surface motility but were unable to respond to a step-down in light intensity. Both positive and negative phototactic responses were abolished in all che mutants, including the cheY mutant. These results indicate that eubacterial photosensory perception is mediated by light-generated signals that are transmitted through the chemotaxis signal transduction cascade.