It is shown that elastic-plastic response of metal matrix composites reinforced by aligned continuous fibers can be described in terms of two distinct modes. In the matrix-dominated mode, the composite deforms primarily by plastic slip in the matrix, on planes which are parallel to the fiber axis. In the fiber-dominated mode, both phases deform together in the elastic and plastic range. Constitutive equations are derived for the matrix-dominated mode of deformation in composites with elastic- perfectly plastic matrices. Response in the fiber-dominated mode is approximated by the self-consistent and Voigt models. The two deformation modes give different branches of the overall yield surface which identify the state of stress that activates a particular mode, and indicate the conditions for mode transition in a given composite system. The matrix- dominated mode is found to exist in systems reinforced by fibers of large longitudinal shear stiffness, such as boron or silicon carbide. Systems reinforced by more compliant fibers, such as graphite, appear to deform exclusively in the fiber-dominated mode. The results show good agreement with experimental data, and with prediction obtained from a more accurate material model. They also help to reconcile several different plasticity theories of fibrous composites, and suggest limits of their validity.