A sensitivity study is conducted to assess the effect of particle shape on the optical properties of size-shape distributions of randomly oriented prolate spheroidal particles. The emphasis is on covering a large range of the spheroids' aspect ratio in order to include the effects of highly aspherical particles. A modified separation of variables method is used to compute the optical properties of an ensemble of randomly oriented particles of different sizes and shapes. The investigation shows that the elements F11, F22/F11, and F34/F11 of the Stokes scattering matrix are very sensitive to a variation in the effective aspect ratio of the equiprobable shape distribution considered here. This implies that the effective aspect ratio will have significant impact on the radiance, the degree of linear polarization, and the degree of circular polarization of light multiply scattered by such an ensemble of particles. The same elements are only mildly sensitive to a variation in the effective variance of the shape distribution. The remaining elements of the scattering matrix are moderately sensitive to a variation in the effective aspect ratio, and rather insensitive to a variation in the effective variance. The single-scattering albedo and the asymmetry parameter are found to be near-linear functions of the effective aspect ratio and rather insensitive to the effective variance. The results indicate that knowledge of the effective aspect ratio might be sufficient to characterize the single-scattering albedo and the asymmetry parameter of the shape distribution. ¿ 1999 American Geophysical Union