The structure of Daphnia pulex heart was studied by means of light and electron microscopy. The heart wall, for the most part, is one cell thick, with occasional regions being four or five cells in thickness. These cells are quite thin and contain many giant mitochondria, long striated myofibrils, an abundance of sarcoplasm, and an irregularly indented cell surface. The striated myofibrils are composed of filaments of two sizes, 50 to 60 A and 120 A, the former forming A and I bands with a prominent Z band but no H or M bands, and the latter being limited to the A bands.