In this study the switchgrass (Panicum virgatum L.), a biomass crop being developed in North America and Europe, was tested as a stiffening and reinforcing agent in polypropylene (PP) composites with and without maleic anhydride grafted PP (MAPP) as a compatibiliser and to evaluate the effect of pulping and different sources of switchgrass on composite characteristics. The refiner pulping yield for two switchgrass varieties was estimated between 70-80%. The addition of 30% (by weight) switchgrass pulp resulted in an increase of the flexural modulus by a factor of about 2.5 compared to pure polypropylene. Which was only slightly lower than values found for jute and flax. The flexural strength of PP composites reinforced with pulped switchgrass and MAPP was almost doubled compared to pure PP and approached values found for jute and flax. The compatibilising effect of MAPP has been visualised by micrographs. The good mechanical properties are achieved despite the severe fibre length reduction as a result of thermoplastic compounding which is shown by fibre length analysis. The impact strength of switchgrass/PP composites was much lower than for pure PP. The use of different switchgrass varieties and harvesting time had a minor to no effect on the mechanical performance of the respective composites. The chemical composition of different varieties was fairly constant. The low price and the relatively good mechanical characteristics should make switchgrass an attractive fibre for filling and stiffening in thermoplastic composites. Further improvement of composite mechanical properties should be possible. (C) 2003 Kluwer Academic Publishers.