a method is provided for controlling the molecular weight of a polyhydroxyalkanoate containing at least one of a unit : — m r 1 ) ch 2 c ]— , and a unit : — k c 6 h 1 ) r 2 ) ch 2 c ]— , wherein a microorganism is cultivated in the presence of a hydroxyl group - containing compound , which is capable of producing the polyhydroxyalkanoate from r 3 q ch 2 ch 2 cooh or r 4 c 6 h 10 r ch 2 ch 2 cooh .