A process for producing poly-.beta.-hydroxybutyrate (PHB) involves contacting maple sap with bacteria (e.g Alcaligenes latus) that convert sugars into poly-.beta.-hydroxybutyrate. The use of maple sap is comparable to or better than the use of pure sucrose, and the use of maple sap results in PHB having higher weight average molecular weight than PHB produced from pure sucrose, making maple sap-derived PHB more suitable for commercial utilization.