Dynamic tensile modulus and X-ray scattering measurements were made for a series of bacterially produced poly- beta -hydroxybutyrate/ beta -hydroxyvalerate (PHB/HV) random copolymers. The position of the primary relaxation is shifted to lower temperatures by increasing HV content and the material becomes generally softer. Drawn PHB/17% Hv copolymer shows conventional mechanical anisotropy. Results are presented comparing the dynamic mechanical behavior of isotropic PHB/HV with that of conventional thermoplastics such as polyethylene terephthalate, isotactic polypropylene and polyethylene.