Tailoring the mechanical properties of melt-spun biopolyester fibers is essential in order to meet the high demands of certain applications e.g. in biomedicine, filtration, biotextiles. Some applications demand e.g. bio-degradable shock-absorbing ductile textiles, while others demand high-tenacity fibers. Mechanical properties of melt-spun fibers are influenced by e.g. different drawing or annealing procedures. Stress, temperature and drawing strongly affect the structure and thus also the mechanical performance. This way it is possible to obtain e.g. ductile fibers that have viscoelastic properties, or fibers that are rather brittle but have high tensile strength. The structure of such biopolyester fibers can be studied in-situ with wide-angle x-ray diffraction (WAXD) and small-angle x-ray scattering (SAXS). These techniques are typically used to obtain information about the crystallinity, crystal sizes, crystal orientations and distances between lattice planes. Additionally, WAXD can be used to obtain information about highly oriented non-crystalline mesophases that are present in certain semi-crystalline polymer fibers. In the past, it has been shown that mesophases can strongly influence the physical properties of melt-spun fibers. We have analyzed the structural response of melt-spun poly-3-hydroxybutyrate (P3HB) fibers to stress, temperature and drawing. The focus lies on the changes that the mesophase and crystals undergo during drawing and annealing and their impact on the mechanical properties. Other examples of biomaterials with mesophases will also be given.