A first order continuum theory with microstructure is developed for aligned short-fiber composites. The fibrous material is modeled by a triply periodic array of rectangular parallelpiped elastic fibres which are embedded in an elastic matrix. A proper reduction of the theory, by which the microstructure variables are eliminated, yields the effective moduli of the short-fiber composite. The overall elastic constants of the three specific situations of long-fiber composites, particulate composites and periodically bilaminated media, are obtained as special cases. The reliability of the predicted effective moduli is verified by numerous comparisons with available experimental results in various cases.