The tick Ixodes ricinus may carry microorganisms which cause serious human and animal diseases, i.a., the Lyme disease (borreliosis), caused by the spirochaete Borrelia burgdorferi sensu lato and babesiosis, induced by the protozoan Babesia microti. Both microbe species may co-occur in the same and other species of the genus tick and produce a mixed infection in humans and animals. The major objective of the study was to identify DNA of B. burgdorferi and B. microti in the I. ricinus ticks collected in spring and autumn 1999 from 6 sites in north-western Poland. The microbial DNA was identified with polymerase chain reaction (PCR). The marker used to detect the B. burgdorferi s.l. DNA was a fragment of the fla gene encoding the protein flagellin, while the B. microti DNA was detected with a fragment of the gene encoding 16S rRNA. A total of 550, 1,160, and 385 tick adults, nymphs, and larvae, respectively, were examined. Among the 155 (7.4%) B. burgdorferi- infected ticks and the 130 (6.2%) infected with B. microti, mixed infection was detected in 0.6% of individuals. The prevalence of coinfection differed between the tick developmental stages. Coinfection was most prevalent (3.1%) in females, males and nymphs being less affected (0.4 and 0.2%, respectively). No coinfection was revealed in the tick larvae. The study described was the first of its kind to be conducted in the former District of Szczecin. For the phenomenon of microbial co-occurrence and related mixed infections to be properly evaluated, the research will be continued.