The effects of wood fibers on the melt rheological behavior of isotactic poly(propylene) (i-PP)/wood fiber (WF) composites have been studied at WF concentrations of 0-32.2 vol % at 493 K. Shear stress-shear rate variations obeyed a power law equation, and the composites exhibited shear thinning, which increased with filler content. At a low shear rate, the apparent melt viscosity increased, while melt elasticity, after an initial decrease, also increased with WF concentration. At a higher shear rate, after an initial decrease, the melt viscosity showed an increase, as did melt elasticity, with increase in filler content. A titanate coupling agent, LICA 38, used to modify the wood fiber surface, modified these rheological parameters by functioning as a plasticizer/lubricant. (C) 2003 Wiley Periodicals, Inc.