The roles of two species of rodents: the bank vole (Clethrionomys glareolus) and the yellow-necked mouse (Apodemus flavicollis) in maintaining the vector Ixodes ricinus and Borrelia burgdorferi s.l. were studied. More particularly, seasonal patterns of rodent abundance and infestation rate with I. ricinus ticks as well as the prevalence of B. burgdorferi s.l. and the two species B. garinii and B. afzelii were determined. The studies were carried out in woodlands at Urwitałt near Mikołajki on the Mazury Lakes in 2001. A total of 196 rodents were trapped and 1349 I. ricinus ticks were collected. There was an increase of rodents in the autumn, with a maximum density of C. glareolus in October and in of A. flavicollis in September. The infestation rate of the investigated animals was above 87% and was dependent on the developmental stage of ticks and host species. Only two stages of I. ricinus were found on trapped rodents: larvae 80% and 84.5%, and nymphs 7.5% and 15.5% for C. glareolus and for A. flavicollis, respectively. The average level of infestation with ticks on rodents appears to be seasonally dependent, and decreases from spring to autumn. Both methods of B. burgdorferi s.l. detection, PCR and IFA, showed that larvae collected from both species of infested rodents were infected by these spirochetes. An increase in the infection rates with B. burgdorferi s.l. was observed from spring to autumn from 12.5% to 15.6% for larval ticks collected from C. glareolus, and from 5.6% to 8.8% for ticks collected from A. flavicollis. The infection rates of larvae examined by PCR were 5% in the spring and 8% in the autumn for ticks collected from C. glareolus, and 10% and 5% for ticks collected from A. flavicollis. A total number of 329 ticks were analysed for the detection of genospecies (184 for B. garinii and 145 for B. afzelii). In larvae collected from C. glareolus, B. garinii were found in 3% and B. afzelii in 1.1% of assessed ticks. Conversely, in larvae collected from A. flavicollis the percentage of infected ticks was 1.5% for B. garinii and 1.7% for B. afzelii. The examined ear biopsies were Borrelia positive in 2.5% and 4.3% of the 44 bank voles and 80 yellow-necked mice, respectively. Both species of rodents, highly abounded in the Mazury Lakes district od North-Eastern Poland, may represent significant, seasonally dependent, zoonotic reservoirs of B. burgdorferi s.l.