Four commercially available samples of bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] containing from 6.5 to 21.8 mol % 3HV have been fractionated by chloroform/n-heptane mixed solvent. They have been separated into several fractions with wide composition ranges. These results show that the composition distribution of as received bacterial P(3HB-co-3HV)s is extremely broad and/or has many peaks over a wide composition range. The effects of the complex composition distribution on physical properties have been analyzed through the comparison of melting and crystallization behavior between samples before and after fractionation. Three as received P(3HB-co-3HV)s show the behavior corresponding to the average composition in spite of their complex composition distribution. The melting temperature and spherulite growth rate correspond to the values expected from extrapolation of the data from the fractionated samples. In these copolyesters, cocrystallization of the chains within the wide composition range occurs. However, one as received P(3HB-co-3HV) has a higher melting temperature and faster growth rate than might be expected. The apparent crystallization behavior corresponds to that of P(3HB-co-3HV) with lower 3HV content. These data suggest that only the component chains of relatively low 3HV content are crystallized in this as received P(3HB-co-3HV). The crystallization of components of high 3HV content is significantly restricted. The extremely broad composition distribution of this copolyester affects the crystallization (melting) behavior.