Environmental factors may drive tick ecology and therefore tick-borne pathogen (TBP) epidemiology, which determines the risk to animals and humans of becoming infected by TBPs. For this reason, the aim of this study was to analyze the influence of environmental factors on the abundance of immature-stage Ixodes ricinus ticks and on the prevalence of two zoonotic I. ricinusborne pathogens in natural foci of endemicity. I. ricinus abundance was measured at nine sites in the northern Iberian Peninsula by dragging the vegetation with a cotton flannelette, and ungulate abundance was measured by means of dung counts. In addition to ungulate abundance, data on variables related to spatial location, climate, and soil were gathered from the study sites. I. ricinus adults, nymphs, and larvae were collected from the vegetation, and a representative subsample of I. ricinus nymphs from each study site was analyzed by PCR for the detection of Borrelia burgdorferi sensu lato and Anaplasma phagocytophilum DNA. Mean prevalences of these pathogens were 4.0% ±1.8% and 20.5% ±3.7%, respectively. Statistical analyses confirmed the influence of spatial factors, climate, and ungulate abundance on I. ricinus larva abundance, while nymph abundance was related only to climate. Interestingly, cattle abundance rather than deer abundance was the main driver of B. burgdorferi sensu lato and A. phagocytophilum prevalence in I. ricinus nymphs in the study sites, where both domestic and wild ungulates coexist. The increasing abundance of cattle seems to increase the risk of other hosts becoming infected by A. phagocytophilum, while reducing the risk of being infected by B. burgdorferi sensu lato. Controlling ticks in cattle in areas where they coexist with wild ungulates would be more effective for TBP control than reducing ungulate abundance. ; We acknowledge funding from the Principado de Asturias (CSIC agreement) and the Castilla-La Mancha (POII09-0141-8176 and EU-FEDER). F. Ruiz-Fons was funded by the Instituto de Salud Carlos III of the Spanish Ministry of Science and Innovation (MICINN) and by the Spanish National Research Council (CSIC). I. G. Fernández-de-Mera was funded by the European Social Fund and the Junta de Comunidades de Castilla-La Mancha (program FSE 2007–2013). P. Acevedo was supported by the Vicerrectorado de Investigación of the University of Málaga and partially by project CGL2009-11316 of the European Social Fund. ; Peer Reviewed