The study was aimed at determining the cause of a disease in a horse exhibiting symptoms of fever, joint effusion, weakness, and extravasations on the mucous membranes. Blood was drawn from the animal for haematological and biochemical molecular tests. The PCR technique revealed the presence of 16S RNA Ehrlichia spp. genetic material in the blood samples. DNA amplification by means of primers EHR 521 and EHR 747 gave a product with a volume of 247 bp.The sequence of the PCR product obtained showed a 97.6% similarity with a sequence of a fragment of 16S RNA Ehrlichia phagocytophila, gene number EU 090186 in the GenBank. A microscopic examination of blood smears demonstrated the presence in the neutrophiles of intracytoplasmatic structures characteristic for Ehrlichia.The treatment involved the intravenous application of oxytetracycline at a dose of 8 mg/kg of body mass for 7 days, and resulted in a gradual recovery. The positive results of the molecular test and the positive reaction to the therapy are an indication of the first case of ehrlichiosis in a horse in Poland as confirmed in laboratory test.