Sera of 500 inhabitants of north-eastern Poland, 450 suspected for Lyme borreliosis and 50 blood donors (control group), were analysed for the presence of IgG antibodies against Anaplasma phagocytophilum, the causative agent of human anaplasmosis (HA), known so far as human granulocytic ehrlichiosis (HGE). Forty one (9.1%) sera of the study group and one serum (2%) of the control group were positive using indirect fluorescence assay (IFA). The seropositivity tended to be more frequent among males (10.3%) than females (7.6%) and among the rural (10.3%) than urban population (7.5%); however, differences were of no statistical significance (p = 0.4). No age difference was found between the seropositive and the seronegative individuals (p = 0.77). The only factor increasing the risk of HA seropositivity found was forestry employment (p < 0.05). Additionally, a total of 559 Ixodes ricinus ticks, collected in the same area as sera, were investigated for the presence of A. phagocytophilum by the polymerase chain reaction (PCR) and 41 (8.7%) of them were found to be positive. The infection level ranged from 2.3–13.7%, depending on the area studied. Bacteria were significantly less frequently detected in nymphs - 2.1% (5/235) than in adult ticks - 13.6% (44/324) and in males - 4.2% (74/165) than in females - 23.3% (37/159) (p ≤0.05). The obtained results confirm both the occurrence of HA foci in north-eastern Poland with I. ricinus as the principal vector of the A. phagocytophilum infection, and forestry workers as the main group at risk.