Structure, phase transitions and mechanical properties of fibers of biodegradable bacterial polyester poly(beta-hydroxybutyrate) and its random copolymers, prepared by gel-spinning and multistage melt-extrusion, were studied by X-ray diffraction, DSC and mechanical testing. It was found for the first time that as-spun fibers of the homopolymer display pronounced hard-elasticity. They exhibit reversible recovery of sample dimensions on loading and unloading, a behavior typical of hard-elastic fibers. It was also observed that elastic drawing leads to reversible formation of the strain-induced columnar mesophase with a 2D pseudohexagonal arrangement of conformationally disordered chains in addition to the orthorhombic crystalline and amorphous phases of the initial material. In contrast, binary and ternary random copolymers based on poly(beta-hydroxybutyrate), being still crystallizable in spite of having relatively high comonomer content, behave like rubbers rather than true thermoplastics. (c) 2006 Elsevier Ltd. All rights reserved.