The effect of bitumen concentration and mean drop diameter and distribution on the rheological behavior of bitumen in water emulsions was investigated. Results obtained showed that the emulsion viscosity is severely affected by the type of drop size distribution. In fact, emulsions with a bimodal drop diameter distribution exhibit viscosity values which were at least 1 order of magnitude lower than the viscosity of similar emulsions, having the same bitumen concentration and an equivalent mean drop diameter, but with a unimodal distribution. It was also found that concentrated bimodal emulsions behave like Newtonian fluids, while the unimodal counterparts are non-Newtonian and show some pseudoplasticity. Because of their very simple rheological behavior, bitumen in water bimodal emulsions are of great importance in the processes of transporting, handling, and commercializing extremely viscous hydrocarbons.