AbstractThe viscosities of fat crystals extracted by 1 % Aerosol O.T. from butter and margarine have been studied using a Weissenberg Rheogoniometer. By applying the Brodnyan (1959) equation for non‐spherical particles to the viscosity data at high rates of shear, after correcting for secondary flow phenomena, it was deduced that the axial ratios of the margarine fat crystals were between 3.05 and 3.94, with the fat crystals from butter lying midway between the two extremes. By applying viscosity equations of the exponential type to such data, it should be possible to calculate the viscosity of butter and margarine at very high rates of shear when the viscosity of the liquid oil in the fat phase and the volume concentration of water drops are known. This information could be useful for predicting the spreadability characteristics of butter and margarin