A method is provided for controlling the molecular weight of a
polyhydroxyalkanoate containing at least one of a unit:
--[OCH((CH.sub.2).sub.m R.sub.1)CH.sub.2 C(O)]-- (m=1-8; R.sub.1 is a
residue having a ring structure of any of phenyl and thienyl structure),
and a unit: --[OCH((CH.sub.2).sub.k C.sub.6 H.sub.1) R.sub.2)CH.sub.2
C(O)]-- (k=0-8; R.sub.2 denotes a substituent on the cyclohexyl group
including H, CN, NO.sub.2, halogen atom, CH.sub.3, C.sub.2 H.sub.5,
C.sub.3 H.sub.7, CF.sub.3, C.sub.2 F.sub.5 or C.sub.3 F.sub.7), wherein a
microorganism is cultivated in the presence of a hydroxyl group-containing
compound, which is capable of producing the polyhydroxyalkanoate from
R.sub.3 (CH.sub.2).sub.q CH.sub.2 CH.sub.2 COOH (q=1-8; R.sub.3 contains a
residue having a ring structure of phenyl or thienyl) or R.sub.4 C.sub.6
H.sub.10 (CH.sub.2).sub.r CH.sub.2 CH.sub.2 COOH (r=0-8; R.sub.4 denotes a
substituent on the cyclohexyl group including H, CN, NO.sub.2, halogen,
CH.sub.3, C.sub.2 H.sub.5, C.sub.3 H.sub.7, CF.sub.3, C.sub.2 F.sub.5, or
C.sub.3 F.sub.7).
