Rheological properties of a series of microbially synthesized poly(3-hydroxybutyrate-co-4-hydroxybutyrate)s, P(3HB-co-4HB)s, with different molecular weights and 4HB compositions have been systematically investigated. Capillary and oscillatory shear measurements have been carried out to characterize the flow behavior of these biopolyesters as a function of temperature at different flow conditions. The rheological. characteristics of these samples show that the 4HB content does not appear to strongly affect the critical molecular weight Me for chain entanglement. Under low stresses during creep measurements, the shear viscosity of a sample with low 4HB content diverges abruptly in a narrow temperature range due to the polymer crystallization. The same sample can still undergo capillary flow below the melting temperature at high stresses where the sample yields. Apart from the creep measurements, the crystallization behavior of the semicrystalline sample has been further characterized using stress-controlled oscillatory shear measurements during a cooling-heating cycle at a constant rate of temperature ramping. The rapid increase and decrease of the dynamic viscosity and storage modulus are interpreted as corresponding to crystallization and melting, respectively, during the thermal cycle. These rheologically determined crystallization and melting temperatures have been found to be ca. 20 degreesC apart and over 30 degreesC higher than those indicated by the DSC measurement.