As a precursor to optimization of processing conditions in staple yarn spinning, 50,000 cotton fibers have been tested on Mantis® single fiber tester. The fibers were sampled from bales of over 40 American upland cottons in order to study the effects of repeated processing on the tensile behaviors of the cotton. Of particular significance was the effects of single fiber breaking elongation on bundle strength efficiency and spun yarn strength. The complex features of the bivariate distributions of elongation and strength are directly translated into the tensile properties of the resulting yarns and fabrics.