To aid in the development of biomass fuels based on waste animal fats, as of yet unpublished theological properties of these fats and their mixtures with petroleum-based fuels are needed. The viscosities and densities of beef tallow, choice white pork fat, poultry fat and yellow grease were experimentally determined to examine their potential use as fuel substitutes or additives. The viscosities of 30% mixtures of these fats with #2 fuel oil fuel were also examined. Dynamic viscosities of these agricultural oils were measured for shear rates of 0.65-32.34 s(-1) at temperatures between 54.4degreesC and 85degreesC. The resulting measurements were fitted to a power law model to obtain values for the consistency coefficient and the flow behavior index. These results indicated pseudoplastic flow behavior for all products, with increasingly Newtonian behavior at higher temperatures and shear rates. The mixtures, of these fats with fuel oil exhibited even more encouraging theological properties very near those of pure fuel oil.