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Abstract Pathogens affecting Antarctic fishes remain mostly unknown and are largely limited to the description of macroparasites such as leeches and endoparasitic worms. Fish, however, occupy a crucial role in the functioning of the Antarctic ecosystem and deterioration of their health can alter the entire Antarctic food chain. In recent years, several studies have identified novel viruses and unicellular parasites affecting the health of notothenioid fishes. Among those, the unicellular parasitic family Xcellidae has received attention following the discovery of an unprecedented disease outbreak in a fjord on the Western Antarctic Peninsula. This pathological situation was caused by a novel X-cell genusNotoxcellia. Soon thereafter, an additional X-cell genus,Cryoxcellia, was described infecting the Bald NotothenTrematomus borchgrevinkiin the Ross Sea. These studies raised awareness and drew observers’ and researchers’ attention to pathologies in Antarctic fishes. Here, we report that during a 2023 Ross Sea shelf survey, a specimen of the Scaly NotothenTrematomus loennbergiidisplaying skin lesions reminiscent ofNotoxcelliainfection had been ingested by an Antarctic ToothfishDissostichus mawsoniand was recovered from its stomach. Molecular analyses confirmed the presence ofNotoxcelliasp. X-cell parasites in the fish’s lesions. This new case of X-cell disease suggests thatNotoxcelliaspp. may have a circumpolar distribution and stresses the need for monitoring Antarctic fish health similar to surveillance protocols for Antarctic birds and marine mammals.