In 2001-2004, four pairs of Barn Owls breeding in nestboxes were visited frequently in order to collect fresh pellets and data on clutch and brood size. The nestboxes were situated in farms on organic soils overlying loamy material in streams of Dwingelerstroom (Dwingeloo and Eemster) and Vledder Aa (Vledder, Doldersum). The surrounding farmland is small-scale (except at Eemster), mostly conventional grassland with some arable fields (near Doldersum including an ecological arable farm), with a varying area surface of woodlots and woodland (1-34% of 100 ha), a low density of houses and 0.68-3.85 km of hedgerows/square km. The 994 pellets revealed 3901 prey items, of which 98.7% mammals, 0.8% birds and 0.5% frogs. Mammals constituted 12 species, of which Microtus arvalis (35.1% in numbers), Sorex araneus/coronatus (20.8%), Apodemus sylvaticus (17.6%) and Crocidura russula (13.5%) were most impartant (in body mass, respectively 44.5%, 8.9%, 20.0% and 8.2%). The proportion of Microtus arvalis varied considerably between individual diets, and was highest at Dwingeloo (53% in biomass) and lowest at Doldersum (33%). The proportion of Microtus arvalis in diets correlated with breeding success (number of young produced per pair), being highest when the diet contained many Common Voles. Barn Owls breeding in the streambed of Dwingelerstroom on average had to cope with larger fluctuations of Microtus arvalis than those of the Vledder Aa (the latter consistently low, as also found in a monitoring scheme for Common Voles; this may have been an artifact of a low in numbers in 2000-03). The study was of too short duration to unequivocally show differences in breeding success between Dwingelerstroom and Vledder Aa.