Azotobacter vinelandii UWD produced very high molecular weight (MW) (approx. 4 million Daltons) poly-beta-hydroxybutyrate (PHB) when grown in 5% w/v beet molasses medium. The polymer MW decreased as the beet molasses concentration was increased. Similar results were obtained in equivalent concentrations of sucrose (as raw sugar), but the polymer MW was not greater than 1.6 million. This difference was not caused by more severe oxygen-limitation in the beet molasses medium. It appeared that the non-sugar components of beet molasses promoted the formation of higher MW polymer. Fish peptone, a known PHB-yield-promoter in this organism, did not promote the formation of very high MW polymer.