An extensive experimental study of structure development during the melt spinning of polypropylene and is as-spun polypropylene filaments is reported. Five polymers representing different molecular weights and polymerization methods were studied. WAXS, SAXS, and birefringence measurements were used to characterize the structure of the filaments. Spinning through air gives rise to monoclinic crystalline structures and spinning into cold water, the paracrystalline smectic form. Both crystalline and amorphous orientation factors were found to correlate with spinline stress for the different polymers studied. Mechanical properties of as-spun fibers such as modulus, yield strength, tensile strength, and elongation to break also correlate with spinline stress.