The understanding of the localization of fillers in polymer blends is of primary importance for the production of polymer blends with controlled properties. In this paper, the effect of blend viscosity ratio on filler localization was investigated by preparing polypropylene (PP)/poly-epsilon-caprolactone (PCL) co-continuous blends with different viscosity ratios K = eta(PCL)/eta(PP), varying from 0.06 to 14.7 and filled with carbon black particles (CB). Polymer phases were premixed before adding CB filler in order to avoid particles incorporation in the phase that melts first (i.e. PCL). The blend co-continuity was checked by solvent extraction technique. The filler localization was studied by different microscopies: Scanning Electron Microscopy, Transmission Electron Microscopy and Optical Microscopy. It was found that fillers located in the highest viscosity phase, whatever its chemical nature (PP or PCL). For viscosity ratio close to one, fillers were localized at the blend interface. These experimental observations were interpreted in terms of hydrodynamic forces acting on filler particles. These drag forces depend on phase viscosity and tend to extract fillers from the interface towards one of the polymer phase. So, fillers are extracted to the highest viscosity phase, applying the most important force. When viscosity ratio is close to one, the two forces balance each other, leading to a "stabilization" of the particles at the blend interface. (C) 2015 Elsevier Ltd. All rights reserved.