Inhibition of bioluminescence in Photobacterium phosphoreum by cerulenin has been demonstrated to be due to a specific inactivation of the acyl-CoA reductase subunit of the fatty acid reductase complex required for synthesis of the aldehyde substrate for the luminescent reaction. In contrast, the activities of the other luminescence-related enzymes, acyl-protein synthetase, acyl-transferase, and luciferase, were unaffected by cerulenin. Myristoyl-CoA, but not NADPH, protected the acyl-CoA reductase against cerulenin inhibition. Cerulenin blocked the acylation of the reductase with myristoyl-CoA and the reaction with N-ethylmaleimide. A shift in mobility of the reductase polypeptide on sodium dodecyl sulfate - polyacrylamide gel electrophoresis occurred after reaction with cerulenin, a shift which could be blocked by reaction with N-ethylmaleimide. These results demonstrate that cerulenin blocks aldehyde synthesis by covalent reaction with the acyl-CoA reductase and indicate that the reaction may occur at a cysteine residue involved in the formation of the acyl-reductase intermediate.