A conspicuous yellow tumour was found for the first time in January 1989 among hooknose Agonus cataphractus collected from different parts of the German Wadden Sea. Prevalence of the tumour was 0.7 %. The tumours arose from connective tissue and were classified as fibromas or possibly fibrosarcomas. Electron microscopy revealed the presence of lentivirus-like particles within cytoplasmic vacuoles of tumour cells. This is the first report of a lentivirus in a lower vertebrate or from an animal residing in the aquatic environment.