A systematic analysis of the melt rheological behavior of a commercial starch-based (MaterBi(R)) matrix composite reinforced with short sisal fibers is presented. The effects of shear rate, temperature, fiber content and treatment were analyzed by parallel-plate rheometry, and classical non-Newtonian models were applied to analyze the pseudoplasticity behavior of the molten composite systems. It is reported that shear rate is the most influential processing condition, while, from the point of view of the material structure, the intercalation effectiveness of the matrix in the fibers is directly linked to the rheological behavior. In fact, processing techniques with high stresses and more efficient mechanical mixing promote the opening of fiber bundles, increasing the aspect ratio of the fibers and the average viscosity of the molten composite. A similar effect on the increase of the aspect ratio and composite viscosity is observed when treated fibers are used. (C) 2004 Society of Plastics Engineers.