To enhance biodiversity, guidelines for farm-based nature management recommend that farmers manage at least 5% of their land as semi-natural habitat, in tandem with other measures. Semi-natural habitats are defined here as those areas of a farm that are non-productive, i.e. areas with no intentional inputs of pesticides or nutrients and remaining effectively undisturbed. Reliable information on the actual amount of semi-natural habitat on arable farms was lacking, however. To address this lacuna the absolute and relative area of such habitats was assessed on 105 arable farms in seven regions of The Netherlands. The results on the national level, showed that on conventional Dutch arable farms 2.1% of the holding was managed as semi-natural habitat, a disappointing figure. No marked interregional differences were found, reflecting the high intensity of land use throughout the country. On average, 1.7% of farm holdings were taken up by farmyards, buildings and farm roads and 96.2% by cropped land. Ditch banks were the most common semi-natural habitat on the farms, followed by ditches, hedgerows and dry ditches. Farms participating in field margin projects maintained twice the average figure (5.3 +/- 2.7%) as semi-natural habitat, compared to 2.4 +/- 0.8% without field margins. Comparing conventional with integrated and organic farms revealed no essential difference in the relative amount of semi-natural habitat on each type of farm. (C) 2003 Elsevier Science B.V. All rights reserved.