The influence of thermal history on morphology, melting, and crystallization behavior of bacterial poly(3-hydroxybutyrate) (PHB) has been investigated using temperature-modulated DSC (TMDSC), wide-angle X-ray diffraction (WAXRD) and polarized optical microscopy (POM). Various thermal histories were imparted by crystallization with continuous and different modulated cooling programs that involved isoscan and cool-heat segments. The subsequent melting behavior revealed that PHB experienced secondary crystallization during heating and the extent of secondary crystallization varied with the cooling treatment. PHB crystallized under slow, continuous, and moderate cooling rates were found to exhibit double melting behavior due to melting of TMDSC scan-induced secondary crystals. PHB underwent considerable secondary crystallization/annealing that took place under modulated cooling conditions. The overall melting behavior was interpreted in terms of recrystallization and/or annealing of crystals. Interestingly, the PHB analyzed by temperature modulation programs showed a broad exotherm before the melting peak in the nonreversing heat capacity curve and a multiple melting reversing curve, verifying that the melting-recrystallization and remelting process was operative. WAXRD and POM studies supported the correlations from DSC and TMDSC results. (c) 2005 Wiley Periodicals, Inc.