Photorhabdus temperata is a bioluminescent bacterium that lives in mutualistic association with entomopathogenic nematodes of the genus Heterorhabditis. The bacterium exists in two morphologically distinguishable phases (primary and secondary). The swimming behavior of P. temperata was investigated. Both the primary and secondary variants were able to swim in liquid or semisolid media under appropriate conditions. Variation in the oxygen levels had little affect on the chemotaxis and motility of the primary form, but greatly influenced the behavior of the secondary form. Under oxic conditions the secondary form was nonmotile, but motility was induced under anoxic conditions. Several phenotypic traits of the primary form were not expressed under anoxic conditions. The constituents of the growth media affected the motility of both variants. P. temperata required additional NaCl or KCl for optimum motility and chemotaxis. Optimal chemotactic behavior required the presence of bacto-peptone and yeast extract in the swim-migration medium. A mutant that was isolated from the secondary form was able to swim under oxic conditions and possessed an altered salt requirement for motility.