ABSTRACT: Reared larval and juvenile Atlantic halibut Hippoglossus hippoglossus collected during incidences of mass mortality in 2 hatcheries were studied by light and electron microscopy. A vacuolating encephalopathy and retinopathy as well as endocardial lesions were observed. By transmission electron microscopy isometric, spherical, unenveloped virus particles with mean capsid diameters of approximately 25 nm were detected. The virions were found intracytoplasmically in semicrystalline arrays or as membrane-associated aggregates and single particles. Infection could be observed in neurons, astrocytes, oligodendrocytes, microglia/macrophages, lymphocyte-like cells, vascular endothelium, endocardial endothelium, myocardial myocytes and in the mesothelium of the epicardium. Specific immunolabeling was obtained in affected tissues on immunohistochemistry applying primary antisera against nodaviruses isolated from the striped jack Pseudocaranx dentex and the sea bass Dicentrarchus labrax. We suggest that the mortality was caused by a nodavirus-like agent and that this disease of the Atlantic halibut is closely related to diseases termed viral nervous necrosis (VNN), viral encephalopathy and retinopathy, and fish encephalitis, which have been associated with mass mortality in numerous cultured marine teleost species. In Atlantic halibut, viral endocardial lesions were a constant finding, suggesting that viremia may be an important factor in the pathogenesis of the infection.