Specific granules of heart atrial and ventricular muscle cells obtained from vertebrates, especially Pisces, were investigated by electron microscopy. The heart muscle cells of Pisces were generally smaller than those of other vertebrates, and the ratio of nucleus to cell body was large. The cells could be classified into three types according to the degree of development of cell organella: cells with long myofibrils, many Golgi apparatus and many mitochondria; cells with short myofibrils, a few Golgi apparatus and a few mitochondria, and cells with few myofibrils and a few mitochondria. In Pisces, specific granules were only found in a small population of heart muscle cells and most cells had no granules in contrast to mammals. The distribution of granules was diffuce, and granules did not gather. The mean diameter of these granules was as follows: 124nm (lamprey), 200nm (ray), 110nm (sillago), 130nm (Sebastes), 160nm (flathead), 130nm (goby), 97nm (flounder), 110nm (turbot), 160nm (sea bream), 124nm (gray mullet), 180nm (eel), 110nm (loach), 110nm (ayu), 120nm (carp), 120nm (crucian), 110nm (gold fish), 120nm (Rana), 133nm (Geoclemmys), 146nm (snake), 144nm (chicken), 260nm (rat), 170nm (dog) and 210nm (pig). The number of specific granules distributed in heart atrial muscle cells was large in the rat, medium in Geoclemmys, the snake, Rana and lamprey, and small in the chicken, dog and pig. The number in Pisces was large in gray mullet, medium in flathead, goby, flounder, turbot, sea bream, loach, ayu, carp, crucian, gold fith and eel, and small in ray, sillago and Sebastes. The number of specific granules distributed in heart ventricular muscle cells was large in Geoclemmys, the snake and Rana, medium in lamprey, the chicken and dog, and small in the rat and pig. In Pisces, the number was large in gray mullet and flathead, medium in goby and flounder, and small in ray, sillago, Sebastes, turbot, sea bream, eel, loach, ayu, carp, crucian and gold fish. Specific granules were present in heart atrial muscle cells and heart ventricular muscle cells in all vertebrates studied.