Poly-3-hydroxybutyrate (PHB), a biopolymer of important commercial applications, is found in a wide range of Gram-negative and Gram-positive bacteria and cyanobacteria. The present study has resulted in the identification of PHB in the luminescent marine bacteria, Vibrio harveyi, in spite of it being previously classified as PHB-negative. PHB granules with distinct membranes were detected by electron microscopy after fixation and staining of V. harveyi cells with malachite green. Analyses by gas chromatography, nuclear magnetic resonance, infrared, and ultraviolet spectroscopy clearly established the presence of PHB. The synthesis of PHB in V. harveyi was found to be under cell density regulation with the levels increasing from 0 (< 0.2) to 26 mg of PHB/g of dry cell weight during growth in a manner analogous to the induction of luminescence in this bacteria. Moreover, synthesis of PHB in V. harveyi was shown to be controlled by the lux autoinducer, N-(3-hydroxybutanoyl)homoserine lactone, providing not only a potential link between luminescence and PHB production but also showing that the lux autoinducer acts as a general signal transductant. These results have also extended the role of homoserine lactones in metabolic regulation to include the control of synthesis of potential energy reserves.