The formation of myofibrils in the developing leg muscle of the 12-day chick embryo was studied by electron microscopy. Myofilaments of two varieties, thick (160–170 A in diameter) and thin (60–70 A in diameter), which have been designated myosin and actin filaments, respectively, on the basis of their similarity to natural and synthetic myosin and actin filaments, appear in the cytoplasm of developing muscle cells. There is a greater than 7:1 ratio of thin to thick filaments in these young myofibers. The free myofilaments become aligned in the long axis of the cells, predominantly in subsarcolemmal locations, and aggregate into hexagonally packed arrays of filaments. The presence of Z band material or M band cross-bridges do not appear to be essential for the formation or spacing of these aggregates of filaments. Formation of the Z band lattices occurs coincidentally with the back-to-back apposition of thin filaments. An hypothesis concerning myofibril growth, based on the self-assembly characteristics of the filaments, is presented.