The use of inherently biodegradable polymers is a possibility to reduce the amount of plastic waste that cannot be assimilated by microorganisms. Poly(3-hydroxybutyrate) (PHB) is a thermoplastic polyester produced by bacteria as intracellular carbon and energy storage compound. As such the polyester is sustainable, biocompatible and truly biodegradable by enzymatic activity. Our goal is to develop an upscalable melt-spinning process to produce high-strength PHB fibers, which so far couldn't be established. Previous findings suggest that native PHB requests additives to become an engineering plastic. Primary crystallization has to be fast enough for PHB to withstand drawing forces induced by melt-spinning. Secondary crystallization, that develops large spherulitic structures, has to be hindered to prevent brittleness, poor mechanical performance and conglutination of as-spun fibers. We investigated PHB extracted from bacteria, native and in combination with tri-n-butyl citrate (TBC) as plasticizer and/or boron nitride (BN) as nucleating agent to accelerate crystallization rate. With rotational rheometry and gel permeation chromatography (GPC) we could show the detrimental effect of extrusion on molecular weight, and thus on processability and tensile strength of as-spun fibers. By adding TBC, extrusion temperature could be reduced, which resulted in smaller degradation. Differential scanning calorimetry (DSC) revealed that the plasticizer also hinders secondary crystallization. We modified the draw-off unit of our pilot melt-spinning plant so as to achieve fibers dominated by longitudinally oriented lamellae rather than spherulitic structures. This modified unit enabled a complete primary crystallization of the fibers during drawing, leading to PHB fibers with highly oriented crystalline morphology and acceptable mechanical properties.