The perennial C4 grass Miscanthus x giganteus has been recommended as a suitable biofuel crop for cultivation in England and Wales, and is eligible for planting grants from the UK government. Field trials have shown that the potential productivity of M. x giganteus is good, with sites that are suited to M. x giganteus cropping capable of dry matter yields in excess of 20 t ha(-1) yr(-1). The paper reports information on the long-term yield of M. x giganteus growing at seven sites in England. Significant seasonal differences in yield are seen at most sites and in many instances these are attributable to moisture stress. A simple predictive model of M. x giganteus above ground dry matter yield, both with and without constraints on water availability is developed supported by yield assessment and crop physiological data from the field sites. The model is applied to the seven field sites, in order to explore its predictive capability and limitations. In addition, to explore the temporal and spatial variability of M. x giganteus yield, the model is applied in a GIS framework, using a number of national geographic databases, to derive maps of potential and water limited yield for England and Wales. The average model estimates of above ground dry matter yields at harvest for M. x giganteus on arable land in England and Wales, given water limitation, are in the range 6.9-24.1 t ha(-1) yr(-1) d.m. Modelled inter-annual yield variation, as summarised by the coefficient of variation, ranges between 11 and 74% nationally and increases with decreasing mean yield. (C) 2003 Elsevier Ltd. All rights reserved.