We report Monte Carlo simulations of dipolar hard-sphere fluids with a nonuniform distribution of the dipole strengths or different hard-sphere diameters. Dipolar polydispersity significantly reduces the spontaneous orientational order observed in the ferroelectric phase of monodisperse dipolar hard-sphere fluids at high densities and low temperatures. Equimolar binary mixtures of dipolar hard spheres of different diameters sigma(a) and sigma(b) also exhibit ferroelectric order. However, the order parameters are very dependent on the ratio sigma(a)/sigma(b). Chains of dipolar hard spheres, typical of strongly interacting dipolar hard spheres at low densities and low temperatures, were observed in binary fluids of dipolar and neutral hard spheres of different sizes. (C) 2000 American Institute of Physics. [S0021-9606(00)50909-X].