Abstract.An epidemic of cutaneous tumours occurs in northern pike,Esox luciusL., from the Åland Islands of Finland. Previous studies have shown that the neoplasm is of mesenchymal origin and that it is not of B‐lymphocytic lineage; however, a T‐lympho‐cytic origin could not be totally excluded. Here the authors provide morphologic evidence that the tumour could, in fact, be monocyte derived. The close apposition of the tumour cells with a lack of cell‐cell junctions is in keeping with the view that it is not of epithelial origin. A striking morphologic characteristic of the tumour cells is the presence of groups of lysosomes and large numbers of cytoplasmic lipid droplets. Therefore, the authors suggest that the tumour cells have features of histiomonocytic cells and that the neoplasm might be a piscine analogue of true histiocytic lymphoma in humans. These studies represent the first description of this type of neoplasm