The epidemiology of Lyme disease remains poorly understood in France, as data are very limited. Incidence studies have shown significant regional variation, with high risk levels in Alsace, Lorraine, Limousin, Auvergne, and Rhône-Alpes. Related vector studies show correlations between incidence and the density of infected nymphs, but exceptions are common. In the forests of Île-de-France and Picardy, the introduction of the Siberian chipmunk may be influencing the dynamics of Lyme borreliosis. This chipmunk appears to be a reservoir for the disease; it carries high loads of Ixodes ricinus ticks and, furthermore, exhibits increased tolerance to Borrelia species compared to other reservoirs. Ongoing research aims to clarify the role of this exotic squirrel in the incidence of this disease on the outskirts of the Forêt de Sénart (Essonne), where the chipmunk population is estimated at between 10,000 and 20,000 individuals. (R.A.)