Poly-beta-hydroxybutyrate (PHB) production by a Bacillus cereus M5 strain was determined by a spectrophotometric method in sugarbeet molasses. The use of beet molasses may help to reduce the cost of producing such biopolyesters. The effect of different molasses concentrations (1-5 g molasses/100 mL water) on PHB production by this strain was investigated. It was found that this particular strain produces the highest dry cell mass (0.440 g/L) in 4% molasses concentration. This strain produced higher PHB (73.84% PHB on dry cell mass) in 1% molasses concentration. It was also observed that PHB productivity by this strain decreased by the increasing the molasses concentration.