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INTRODUCTION. Monogeneans of the Family Calceostomatidae Parona & Perugia, 1890 parasitize different species of Sciaenids worldwide: among these, the calceostomatid Ktariella polyorchis Vala & Euzet, 1977 has been described infecting the gills of wild meagre Argyrosomus regius (Asso, 1801) caught off Tunisia and Egypt. The present investigation reports for the first time the occurrence of K. polyorchis in broodstock of A. regius from a Croatian farm. MATERIALS AND METHODS. At the farm, gills samples and skin scrapings were collected from heavily infected fish and preserved in 70% ethanol and 10% buffered formalin. Ethanol-fixed parasites were subjected to morphological analysis following clarification in Amman’s lactophenol. Before clearing the parasites, a section of the body without diagnostic characters was excised with a sterile scalpel and processed for molecular analysis, amplifying and sequencing the complete ITS and partial 28S rDNA. For observation in Scanning Electron Microscopy (SEM), formalin-fixed specimens were dehydrated in a graded ethanol series, critical point dried and sputter coated with gold-palladium. RESULTS AND CONCLUSIONS. Although being reported mainly from the gills, in our samples K. polyorchis were mainly found on the fins and skin, while the gills were infected with another monogenean species, the diplectanid Diplectanum sciaenae Van Beneden & Hesse, 1863; this finding may indicate a possible displacement of K. polyorchis in concurrent heavy infections with D. sciaenae. Originally assigned to the genus Calceostoma Van Beneden, 1958 based on the morphology of the anterior region and of the armature in the haptor, K. polyorchis was subsequently assigned to a separate genus based on the morphology of the male reproductive system. Our study adds further morphological information, providing the first detailed description of the parasite surface by SEM analysis, and the first molecular data, useful in future taxonomical and phylogenetic studies.