A nine-year-old male dog of Doberman breed was taken to the laboratory for animal reproduction of the Scientific Veterinary Institute „Novi Sad“ for semen quality control. The sample of the ejaculate was taken without difficulty; however, the sample contained a significant amount of fresh blood (total ejaculate volume was 5 ml). The assessment of semen quality from such sample was not possible because blood components mask the spermatozoa. Nevertheless, the sample of ejaculate drop was placed for microscopic observation and analysis. In the semen, a lot of blood cells and only a few spermatozoids (mostly not moving) were found; however, something else was observed - the presence of a live motile organism, long-shaped, looking very much like a larval stage of a parasite Dirofilaria sp. Next day, the blood sample was taken for the analysis for dirofilariosis. The result of the ELISA test for Dirofilaria immitis antigen was negative indicating the absence of adult worms of Dirofilaria immitis. The result of modified Knott test showed the presence of larval stages of Dirofilaria repens.