The effects of copolymer composition on structure development in injection molded bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBHV) parts were investigated. The increase of hydroxyvalerate co-monomer content lowers the melting temperature as it disrupts the crystalline order as a result, depth variation of melting behavior in the injection molded samples were found to depend primarily on the co monomer composition. At lower HV concentrations, the skin regions were found to exhibit a single melting peak that is also higher than those in the interior of the parts where generally bimodal melting behavior is observed. At higher HV content bimodal melting prevails throughout the injection molded parts including the skin and shear regions. This unusual behavior was attributed to the flow induced crystallization in extended chain formation at the skin and case of inclusion of HV defects in the HB crystals that formed at slower cooling conditions in the interior creating thinner HV rich crystals with lower melting and thicker HV poor crystals with higher melting peaks. Depth profiling micro beam wide angle X-ray diffraction studies revealed that these polymers exhibit two distinct orientation behaviors depending on the distance from the surface. At the skin, invariably the chain axes are oriented along the flow direction. Beyond the transition layer located between the shear layer and core, the orientation does not disappear as expected from fast crystallizing polymers but rather preferential orientation of crystals with their a-axes along the flow direction was observed. At low HV content, the materials exhibit unusually high preferred orientation behavior throughout the thickness even for thick moldings, resembling the orientation behavior of polymers with low orientation relaxation behavior such as thermotropic liquid crystalline polymers. This is partly attributed to the unusually low injection melt temperature employed in these materials to avoid thermal degradation. The increase of HV content in the copolymers was found to change this c+a type orientation gradient across the thickness to gradual decrease of c-axis oriented crystals. This change was attributed to the decrease of crystallizability with the addition of HV and increasing melt fraction in the melt stream as the overall melting temperature decreases with the increase of HV content. (c) 2005 Elsevier Ltd. All rights reserved.