Bacterial poly(beta-hydroxybutyrate) aud bacterial copolyesters of beta-hydroxybutyric acid with up to 29% of beta-hydroxyvaleric acid were shown to spread reproducibly from chloroform to give monolayers at the air-water interface. Typical limiting areas are 2.4 m(2) mg(-1), corresponding to 0.35 nm(2) per repeating unit. Surface pressures of up to 36 mN m(-1) were reached, corresponding to packing densities of approximately 0.12 nm(2) per repeating unit. The spreading of chiral, water-insoluble biopolyesters to stable monolayers, in which a majority of the repeating units are in contact with the water subphase, has important implications for the structure of polyalkanoate inclusions.