Mugil liza, an economic resource fish species along the South American Atlantic coast, serves as the sole mullet species breeding in Argentina. While encysted parasites in mullets typically target the bulbus arteriosus, this study identifies Ascocotyle sp. metacercariae localized exclusively within the cardiac ventricle of Mugil liza. Thirty juvenile specimens were collected from Samborombón Bay, Argentina, to evaluate the resulting cardiac lesions and associated inflammatory host response through histological, lectinhistochemical, and immunohistochemical analyses. Pathological evaluation revealed marked disruption of the myocardial architecture, characterized by regions of coagulative necrosis, presence of melanomacrophage centers (MMCs), and congested blood vessels containing cellular debris within their lumen. A persistent mononuclear inflammatory infiltrate—comprising eosinophilic granular cells, macrophages, and lymphocytes—was observed surrounding the ventricular cysts and extending into the atrium. Lectinhistochemical characterization identified a shared glycan profile, marked by positivity for Phaseolus vulgaris leucoagglutinin (PHA-L) and Pisum sativum agglutinin (PSA), across monocyte and macrophage lineages. This specific carbohydrate expression highlights the active participation and recruitment of these immune cell populations during the host defense. Overall, these pathological and glycobiological findings demonstrate that Ascocotyle sp. metacercariae act as a chronic immunogenic stimulus, driving and sustaining localized myocardial inflammation in M. liza.