The structural development during the melt spinning and subsequent annealing of polybutene-1 fibers was studied with in situ wide-angle X-ray scattering techniques. The online spinning apparatus consisted of a vertically translating extruder that allowed different distances from the spinneret to the stationary X-ray beam to be sampled. For all take-up speeds examined, phase II crystals mainly were formed, with only a small population of phase I crystals existing. As the take-up speed was increased, the crystallinity also increased, indicating that strain-induced crystallization prevailed. The crystalline orientations observed online were very close to perfect alignment with the fiber axis. In addition, annealing studies were performed to study aspects of the gradual phase II to phase I transformation as functions of time and prior processing take-up speed. This transformation was strongly dependent on the take-up speed. The dependence appears to be connected to local stress enhancement via chains connecting crystallites. The results also seem to indicate that at low take-up speeds (17 mpm) there is a series connectivity of amorphous and crystalline components in the fiber, whereas at greater take-up speeds (100 and 250 mpm), the morphology grows into some type of three-dimensional network, possibly a shish-kebob-type morphology.