Poly(3-hydroxybutyrate) [P(3HB)] and its copolymers with hydroxyalkanoates are naturally occurring thermoplastic materials produced by bacteria. There are many potential uses for these copolyesters owing to their biodegradability and biocompatibility. The physical properties of the copolyesters vary depending on the chemical structure as well as the composition of the comonomers. Usually, we expect, copolymers to have a narrow chemical composition distribution (CCD). Several reports, however, have pointed out that some bacterial copolyesters have broad and/or multimodal CCD. Fractionation based on the chemical composition has also been reported for several bacterial copolyester samples. In this review, the literature concerning CCD and fractionation based on chemical composition is summarized. The width of CCD is approximated based on the data of diad sequence distribution. Generality of the complex CCD in bacterial copolyesters is also discussed.