Observation of five nesting pairs of Short-fared Owls (Asio flammeus) in a 70 km(2) area, le bassin du Drugeon, Pontarlier Plain (Haut-Doubs, France), revealed a connection between their distribution in space and time and the abundance of Common Voles (Microtus arvalis) and Water Voles (Arvicola terrestris). The birds selected sectors where the population densities of grassland rodents were not only the highest but also the most evenly distributed. Mean success of capture attempts (capture rate) was 32 %. Differences between males and between habitats (meadows, pastures, marshes) were not statistically significant. However, number of young departing nest and capture rates achieved by the males were closely correlated (r = 0.89). It seems therefore that, given the homogeneity of prey population, reproductive success was determined by male's hunting performance. Diet was studied from 192 pellets representing 297 prey items. The search for correlations between capture attempts by males and the proportion of the different rodent species in the diet of owl pairs showed that the ratios of Common Voles and Field Voles (Microtus agrestis) were related to the males' hunting effort in the habitat typical of each rodent species (permanent grassland r = 0.97 and marshland r = 0.96 respectively). The ratio of Water Voles in the diet was, however, correlated with the hunting effort in marshland (r = 0.96) though the species was more abundant in grassland