The prevalence of the lungworm, Dictyocaulus eckerti, was studied in a sample of 603 roe-deer (Capreolus capreolus) in the Rhone district of France. The mean prevalence of infection (17%) in deer in a given area fluctuated according to the percentage of the area covered with forest, or lake and river. The density of roe-deer or domestic ruminants, the type of forest and the maximum elevation of the site were not related to the prevalence of infection.