Abstract: More than one hundred sets of direct shear tests were conducted on soils composed of four reference clay minerals and various combinations of the four clay minerals of the Columbia University Clay Mineral Standards Project (A.P.I. 49) to illustrate with quantitative results the influence of clay mineralogy on the stress-strain characteristics of soils and hence the practical importance of clay mineralogy in soil mechanics. The soils tested consisted of various amounts by weight of Bedford Indiana Halloysite, Mesa Alta New Mexico Kaolinite, Santa Rita New Mexico Montmorillonite and Fithian Illinois Illite prepared with distilled water. The experimental results agree quite well with hyperbolic stress-strain relations previously developed by the author. The Hvorslev strength parameters are written as exponential functions of the clay mineral content and arc included in the hyperbolic stress-strain law. The ultimate shear strength of the soils are mathematically expressed in terms of the stress history, confinement, and mineralogy.