Echinococcus multilocularis larvae occurred in 23 (2.44%) of the 943 adult vole rats (Arvicola terrestris scherman Shaw, Mammal Microtidae) collected in the mountanous center of France (Massif Central, departments of Puy-de-Dôme and Cantal, province of Auvergne) and obtained from August 1981 to Aug. 1982 in seven different localities. According to this localities, the average percentage of parasitized voles changes from zero to 4.65%; according to the month of the year, these percentages change from 0.6 to 3.6 in the active sites while unusual peaks can be reached (17.5 in January in the Allanche focus with yearly prevalence of 3%). The favorable focuses remain active the whole year. The different aspects of the anatomo-pathological changes of the livers are described, particularly those due to young larvae. 14 (60%) of the 23 larvae are young, small and infertile hydatids. Only 2 (8.7%) of the 23 are fertile. One of these two larvae seems to degenerate and produces a few protoscolex alive; the other simulates a plurilocular larva of E. granulosus, restricted to a single lobe well delimited by the fibrous adventitia and produces the protoscolex in great number. These observations prove the good receptivity of the vole rat to the local strain of the parasite but seem to attest its relative inaptitude to bring the larva to maturity. The dispersion of the alveolar echinococcosis among the wild animals in Auvergne (8.5 of the foxes parasitized and 2.44% of the vole rats) realised a permanent threat of contamination as regard rural population or people on holiday during the summer. The risk seems to be weak (5 human cases of hepatic alveolar hydatid disease for ten years) but actual.