The luxA and luxB genes of bioluminescent bacteria encode the alpha and beta subunits of luciferase, respectively. Sequences of the luxA and luxB genes of Xenorhabdus luminescens, the only terrestrial bioluminescent bacterium known, were determined and the amino acid sequence of luciferase deduced. The alpha subunit was found to contain 360 amino acids and has a calculated molecular weight of 41,005 Da, while the beta subunit contains 327 amino acids and has a calculated molecular weight of 37,684 Da. Alignment of this luciferase with the luciferases of three marine bacteria showed 196 (or 55%) conserved residues in the alpha subunit and 114 (or 35%) conserved residues in the beta subunit. The highest degree of homology between any two species was between the luciferases of X. luminescens and Vibrio harveyi with 84% identity in the alpha subunits and 59% identity in the beta subunits.