Fatty acid reductase from the bioluminescent bacterium Photobacterium phosphoreum, has been partially purified free of aldehyde reductase activity and with a low endogenous fatty acid content permitting the characterization of the aldehyde product of the reaction. Two aldehyde reductases, both dependent on NADH, were separated by anion-exchange chromatography from the fatty acid reductase activity. The partially purified fatty acid reductase catalyzed the synthesis exclusively of long chain aldehydes from fatty acids in the presence of ATP and NADPH as demonstrated by the conversion of [3H]tetradecanoic acid to [3H]aldehyde. Comparison of the amount of [3H]aldehyde produced with the bioluminescence responses of luciferase to the aldehyde product and standard aldehydes, both with respect to maximum light intensity and luminescent decay, established that tetradecanoic acid has been converted to tetradecanal, the aldehyde of the same chain length. These results are consistent with a mechanism involving activation of the fatty acid with ATP followed by reduction of a fatty acyl intermediate to the corresponding aldehyde.