A constitutive equation for collagen fibers is developed in the form of the ‘quasi-linear’ viscoelasticity law suggested by Y. C. Fung. This mathematical model is used to predict the experimental responses of tests of collagen fiber bundles extracted from the tails of mature rats. All experimental tests were conducted with the specimens immersed in a constant temperature saline bath. From relaxation tests, two material constants were obtained and used to predict the results of constant strain rate tests, hysteresis loops, and sinusoidal tests.