Abstract.The structural changes which occur in the heart, the lateral muscle and the large intestine during the development of cold water vibriosis in Atlantic salmon,Salmo salarL., have been studied by light and electron microscopy. The results strongly suggest a causative relationship between the presence of the fish pathogen,Vibrio salmonicida, and the observed pathological changes. Cell damage was invariably accompanied by the presence of the bacterium. A close relationship was found between the number of bacteria detected in the tissues and the degree of morphological damage. The most severe cell damage was found in areas with a rich blood supply. The earliest sign of damage was detected in the cell coat and cell membrane on the luminal side of the endothelial cells of the capillaries. Later in the development, the bacteria were also found intracellularly in endothelial cells. In moribund fish, the cells in the vicinity of the capillaries were also severely damaged. Bacteria appeared intracellularly in leucocytes even in early stages of the disease. This caused more severe damage to leucocytes compared to erythrocytes, which seldom contained bacteria.