Experiments carried out on aligned carbon fiber-reinforced epoxies published elsewhere show that during tensile-tensile fatigue the slope of the S-N curve, β, is increased by reducing the adhesion between fibers and polymers, and decreased by reducing the internal microstresses in the composite. Also, β is increased by plasticizing the polymer. During fatigue, the flexural modulus decreases at a greater rate than the tensile (Young's) modulus. The Poisson's ratio and the hysteresis loop energy both increase during fatiguing. Circular holes drilled in the specimen become gradually more elliptical, with the major axis at right angles to the stressing (and fiber) direction. Final failure involves extensive splitting, and scanning electron microscopic examination of the fractures reveals powdered polymer present on the fracture surfaces. These results suggest that initial fiber waviness may be important. At the antinodes of the waves, there are transverse stresses which could lead to fiber debonding and splitting of the matrix. Once debonded, the fibers could slide within the debonded regions, giving rise to matrix attrition. The cyclic stressing could then cause the powdered matrix thus formed to vibrate and gradually transfer itself to the inside of the curves of the fiber profiles, increasing the fiber curvature (and hence increasing Poisson's ratio, and the width of the hole in the specimen and accounting for the decline in Young's modulus). Eventually the fiber curvature could increase to such an extent that the resulting flexural stresses, combined with the tensile stresses, are enough to cause fiber failure in the regions where this process has developed farthest. Then final failure could occur by connection of these regions through splits in the matrix. (These splits must also develop continuously during fatigue, to account for the gradual loss in flexural modulus.) The process is accelerated by poor fiber-matrix adhesion, and slowed down by reducing the internal stresses which are present due to the cure shrinkage of the resin.