We investigate the effect of polydispersity in fiber cross section on the effective properties of a fiber‐reinforced composite. For a model microstructure in which the fibrous inclusions are fully penetrable cylinders with varying diameters, we compute the microstructural parameters &zgr;’1and &eegr;1which determine Milton’s upper and lower bounds on transport coefficients and elastic moduli of the composite. From our results we infer that polydispersity in fiber size has very little influence on these properties.