Shear rate and viscosity correction factors for a Casson fluid in cylindrical (Couette) geometries-type I, concentric cylinder with a narrow gap, and type II, cylinder in a semi-infinite medium-are developed in this work. The analytical correction factors compare well with published data corrected by general methods. Corrected viscosities were also compared to viscosity-shear rate data on chocolate in an absolute viscometer (cone-and-plate), which requires no corrections. This comparison showed excellent agreement of the corrected data in the higher shear rate range, but not very good agreement in the lower shear rate range. Reasons for this are discussed. In addition, a Reiner-Riwlin type formulation was developed for determination of model constants directly from rotational viscometry data without the need to correct for non-Newtonian shear rate, but this method is less accurate than existing slope-intercept methods for determining model constants. The analytical correction procedure developed here is expected to be useful wherever the Casson model is used to describe the theology of a fluid, particularly one exhibiting yield-stress pseudoplastic behavior. (C) 2003 Elsevier Inc. All rights reserved.