Our thesis aimed to recognize which intraerythrocytic blood parasites infect two species of Slovene lacertids, common wall lizard (Podarcis muralis), and Horvath's rock lizard (Iberolacerta horvathi). We confirmed the identity of the parasites using molecular approaches by sequencing a part of the 18S rRNA gene. All observed parasite individuals belong to the genus Karyolysus, known by parasitizing reptiles and lacertids specifically. Karyolysus is a genus of protozoan hamogreagrines. Further work included the analysis of prevalence and intensity of parasitic infection in both species of lizards. We compared prevalence and intensity between species, sexes, size, locations, altitudes, and whether the populations live in syntopy or alotopy. We confirmed that the prevalence and intensity of blood parasites were significantly higher in Horvath's rock lizards. We also observed a higher prevalence of parasites in larger individuals, but this was found in only two locations. Contrary to expectations, we found in two localities that the intensity of parasites was lower in larger animals. The intensity was also higher in males. We did not confirm any differences in prevalence or intensity between syntopic and alotopic populations. We also did not observe any differences related to altitude. Our results confirm an asymmetric parasitic prevalence and intensity between the two competing lizard species that may have a potential influence on their competitive relationship and coexistence.