Past research indicated that cotton fiber strength increases as the specimen loading rate increases (i.e., Stelometer and Pressley methods). Only limited literature was published on the characteristics of strength when tested at constant rates of extension. This report describes an instrument designed to test 1/8 gage fiber bundles at constant rates of extension using jaw displacement velocities from .05 to 50 cm/min. A computer was used to analyze the voltage signals from the force and displacement transducers. Pressley jaws were used to clamp the test bundles and a Stelometer vice was used to load them in the jaws. Fiber specimens were selected by a mechanized sampling and combing procedure. Performance of the prototype instrument is described with test results for strength, elongation, modulus and two forms of toughness.