poly - β - hydroxyoctanoic acid , a multi - purpose product , is prepared by means of a method for aerobically fermenting bacteria which can accumulate said acid , wherein a nutrient solution containing a growth control agent is added to the culture medium to maintain a dilution level of about 0 . 050 - 0 . 100 h - 1 . once it has been separated from the culture medium , the biomass is suspended in acetone or isopropanol , and the separated and dried cell mass is dissolved in an unchlorinated inert organic solvent . the desired acid is obtained by evaporating the clear filtrate which is recovered after separating the solid mass in the form of a stable polymeric film .