In this paper the effect of particle shape on the entropy of nonspherical particles adjacent to a plane surface is considered, The subsequent influence on particle retention in field-flow fractionation (FFF) has been estimated. New retention equations for thin rod and disc shaped particles have been derived to cover this steric-entropic region of FFF. As particle size increases relative to the mean cloud thickness, the retention ratio for nonspherical particles is predicted to increase compared to small spherical particles of the same mass. This could result in a significant underestimation of the calculated equivalent spherical diameter (d) by FFF methods. The steric-entropic FFF equations could be used to calculate accurate d values if the large particle dimension is estimated independently (e.g., by microscopy). Alternatively, run conditions could be designed to minimize steric-entropic perturbations to the ideal retention equation.