In order to identify the respective importance of the ecological and biological factors involved in the transmission of Echinococcus multilocularis, we estimated grassland vole intermediate host (Microtus sp. and Arvicola terrestris) population densities, in relation to the diet of the definitive host (red fox, Vulpes vulpes) and with the prevalence of E. multilocularis in the fox population. The study was conducted in the Ardennes, north-eastern France, which is an area with a high incidence of alveolar echinococcosis. Surface index methods showed that Microtus was the most abundant intermediate host in the area. Furthermore, Microtus was present in one-third of the 144 faeces and 98 stomach content samples examined and represented more than two-thirds of the rodent occurrences. Red fox predation on Microtus was significantly correlated with Microtus relative abundance. In contrast, the relative abundance of A. terrestris was very low. This species, as well as Clethrionomys glareolus and Apodemus sp., was little consumed. E. multilocularis prevalence in foxes was determined from carcasses and reached 53% (95% confidence interval 45-61%). Intensity of infection varied from 2 to 73,380 worms per fox, with 72% of the sampled worm burden harboured by 8% of the sampled foxes. The selected explanatory variables (sex, year, age class, health and nutritional condition, and season) failed to predict prevalence rate and worm burden. The high prevalence rate in foxes indicates the possibility of intense E. multilocularis transmission, apart from periods, or in landscapes, favourable to large population outbreaks of grassland rodents.