Polyethylene-filler composites (kaolin, BaSO4, and TiO2) were prepd. by in-situ polymn. using a metallocene catalyst and by extrusion mixing in the melt. TiO2 influences the polymn. process. In-situ polymn. was found to be advantageous with respect to encapsulation, dispersion of the filler particles leading to improved mech. properties, except for TiO2 contg. composites. No significant influence on crystn. was obsd.