Beet molasses successfully replaced glucose as sole carbon source to produce poly-beta-hydroxybutyrate by a recombinant Escherichia coli strain (HMS174/pTZ18u-PHB). The fermentation with molasses was cheaper than with glucose. The final dry cell weight, PHB content and PHB productivity were 39.5 g/L, 80% (w/w) and 1 g/L h, respectively, in a 5 L stirred tank fermenter after 31.5 h fed-batch fermentation with constant pH and dissolved O-2 content.