The tensile properties of filaments have been related to fiber microstructure in numerous studies over several decades. However, there have been relatively few attempts to relate shear modulus to microstructure : most of the work on shear properties of filaments was done over 20 years ago. Since then, there have been advances in instrumentation and polymer technology. We present a review of the literature in these areas. We report the construction of a fully automated torsion balance and its efficacy in an investigation of the relationship between polypropylene (PP! fiber microstructure and shear modulus. This work lays the foundation for a definitive research program into microstructure/shear properties’ interrelationships in fibers. Our torsion test apparatus is an enhanced implementation of the simple torsion balance. Data acquisition and test parameters are completely controlled by a microcomputer. Raw data from digital position encoders is converted into torque-twist data, which is then presented to a statistical program to determine the initial shear modulus of the filament. Torsion testing was performed on six PP fiber samples. It was found that the shear modulus of the filaments increased with an increase in amorphous orientation, but that in a filament with a radially differentiated structure, the relationship is biased by the modulus of the outer portion of the filament.