The present study deals with the ecology of Lyme borreliosis in Northern Poland. The complexity of environmental conditions are: (1) the presence of certain rodent hosts for the Borrelia, (2) the seasonal patterns of abundance and infection rate of I. ricinus in the population of rodents and on vegetation, (3) the prevalence of B. burgdorferi s.l., B. garinii and B. afzelii. The long-term studies were carried out on a monthly basis from April to October (1998-2001) in woodlands at Urwitałt near Mikołajki on the Mazury Lakes. Two methods of spirochate detection were used: indirect immunofluorescence assay (IFA) and polymerase chain reaction (PCR). The dynamics of ticks infestation, differences in infestation rates between years, months, age and sex for two species of rodents (Clethrionomys glareolus, Apodemus flavicollis) were analysed using the software package Statgraphics version 7 and GLIM 4 (statistical system for generalized linear interactive modeling, Royal Statistical Society 1993). We were able to demonstrate that in selected habitat of Mazury Lakes the spirochaetal agent of Lyme borreliosis occurs quite frequently, both in rodents and in vector-I. ricinus. Comparing C. glareolus and A. flavicollis, each rodent species showed different, seasonal and sex dependent, host infection with Borrelia. Due to the courses of prevalence of infection, both for partially engorged ticks and questing ticks, the risk of human infection seems to be highest in spring. The infection rate of engorged larvae was much higher than in questing larvae. We conclude that both species of rodents, highly abounded in the Mazury Lakes district, may represent significant zoonotic reservoir for B. burgdorferii sl., B. garinii and B. aftelii.