This paper contains a general account of about forty examples of tumours, mostly “malignant,” found in eighteen species of fishes, all of them marine except two, a Stickleback and a Gold-fish. In all these cases the affection was a very obvious one, attracting immediate attention from the fishermen who caught the specimens, or leading to condemnation of the fish by market inspectors. In some cases the appearance presented by the fish was grotesque, or repulsive in the extreme. I give a list of the specimens dealt with on p. 447–8: all these represent “cancerous” affections. Many other diseased fishes, obtained in the same ways, have been seen and examined. Malformations of development, or growth; large healing scars due to wounds; repulsive ulcers of unknown origin; parasitic tumours and two cases of undoubted piscine tuberculosis (in Cod) have also been examined. Here, however, I refer only to tumours which exhibit the character of malignancy.