Constitutive equations are derived for the rapid shear flow of a mixture formed from granular solids in a fluid. The stress state within the mixture is considered to be created by the momentum exchange between colliding solids. The constitutive equations describe the shear and normal stresses as a function of the velocity gradient, friction and restitution coefficients of the solids, the fluid drag coefficient for the particle shape and the density of the solid and fluid constituents. The theoretical results agree well with the results obtained from numerous laboratory experiments.