The aim of the investigation was to determine the prevalence of T. gondii among a domestic cat population in an urban area in Olsztyn. In total, 135 serum samples of cats collected in several veterinary outpatient clinics in Olsztyn were examined by direct agglutination assay. The Toxo-Screen DA BioMerieux commercial test detected anti-Toxoplasma gondii IgG antibodies. The results of studies indicated that cats bred under different conditions in the city of Olsztyn have contact with various forms of invasive T. gondii. The high percentage of seropositive results at a 1:40 dilution (65.9%) suggests a past infection, and the high percentage of seropositive cases at a 1:4000 dilution (68,1%) indicates a current or recent toxoplasmosis process. This could indicate that there is a permanent source of T. gondii infection in the habitable environment of the cats. The high percentage of T. gondii seropositive results among domestic cats in Olsztyn proves the presence of circulation of the parasite in the environment.