The first case of a metastasizing hlstologically malignant tumor with fatal outcome in a harbor porpoise Phocoena phocoena is described. According to the World Health Organization (WHO) classification of tumors in domestic animals, thls neoplasm represents a well differentiated scirrhous tubular adenocarcinoma of the stomach, metastasizing by way of veins, lymphatics and by implantation in the peritonea1 cavity, uterus, lymph nodes, and liver The possible significance of environmental carcinogenic agents being implicated in the induction of this neoplasm -in contrast to fish -can only be suggested in this first case of a malignant tumor in a North Sea harbor porpoise.