Liquid-nitrogen quenching was applied to study the enthalpy effect on the stress-induced crystallization of microbial polyesters. Crystallization bands of poly(3-hydroxybutyrate) exhibited the potential to reveal the stress distribution in the melt; while crystallization of poly[(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)] gave shishkebab structures. Polarized-light micrographs confirmed that the enhanced nucleation was attributed to the tensile stress. Furthermore, control of the quenching direction provides a method to direct the lamellar growth.