A field study was conducted to disclose the prevalence of different helminth parasites in the black rat, Rattus rattus, in some regions of Stara Zagora district, Bulgaria. Out of 67 examined rats, 53 (79.1%) were found to be positive. Helminths of three classes were detected such as Cestoda: Hymenolepis diminuta (47.8%), Hymenolepis nana (43.3%), Taenia taeniaeformis larvae (7.5%), Taenia polyacantha larvae (1.5%); Secernentea: Syphacia obvelata (4.5%), Aspiculuris tetraptera (3.0%) and

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... rea: Capillaria hepatica (9.0%), Trichuris muris (3.0%). Of all the investigated rats, 27 (40.3%) were infected by one helminth species and 26 (38.8%) by more than one. Hymenolepis diminuta was a predominant species in cases of single infections (23.9%). The most frequent co-infections were observed by H. nana and H. diminuta (16.4%).