The comonomer compositional distribution and thermal and morphological characteristics were investigated for five bacterially synthesized poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] samples with 3HV content of 45, 49, 70, 80, and 96 mol %. All these samples were fractionated into many fractions with widely different 3HV content by changing solvent/nonsolvent volume ratio of chloroform/n-heptane mixtures. Bacterial P(3HB-co-3HV) samples investigated in this study were found to have broad comonomer compositional distribution. The tendencies of the fractional precipitation of the P((3HB-co-3HV)s with 3HV content lower than (50 mol % and those with 3HV higher than 80 mol % were found to be contrary. The 3HV content dependences of the thermal properties and crystalline structures were investigated for bacterial poly(3-hydroxybutyrate) [P(3HB)] and a series of compositionally well-fractionated P(3HB-co-3HV) samples with 3HV content ranged from 14 to 98 mol % by DSC, WAXD, and solid-state C-13 NMR. It was found that P(3HB-co-3HV) samples with 3HV content lower than about 47 mol % form the crystalline lattice having the P(3HB) homopolymer type lattice including the 3HV unit as the crystal constituent, and those with a 3HV content higher than about 52 mol % form the crystalline lattice having the P(3HV) homopolymer type lattice including the 3HB units. Thus, P(3HB-co-3HV)s show the crystalline structural change in a very narrow range of 3HV content.