Wheat straw is an abundantly available and potentially valuable biomass that is currently underexploited. In this study, the feasibility of using wheat straw as a filler in high-density polyethylene (HDPE)-based composites was explored. Straw was treated with the white-rot fungus Pleurotits ostreatus with the aim of improving adhesion between straw and plastic, and thereby the mechanical properties of the composite. Results indicate that the use of sterilized straw is necessary to inhibit the growth of indigeous organisms that preclude, likely through competition, removal of lignin, and hence, improved bonding between straw and plastic. Light and transmission electron microscopy revealed cell wall modification in sterilized, inoculated straw. Reduced thermal stability of treated straw did not negatively affect the production of injection-molded straw-plastic composites (SPC). Comparable interfacial adhesion, based on activation energies obtained in dynamic mechanical analysis, was observed in untreated and treated straw- and pine-based thermoplastic composites. The results of this study indicate that wheat straw represents a promising alternative to wood fillers in the production of thermoplastic composites. (c) 2006 Wiley Periodicals, Inc.