Perfused Chara cells capable of resuming ATP-dependent cytoplasmic streaming in low free Ca++ solutions have been examined by electron microscopy for myosin-like filaments. Filaments 44 nm in diameter and up to 3 micron in length have been found associated with the endoplasmic reticulum that along with mitochondria, microbodies and dictyosomes from the endoplasm becomes immobilised around the sub-cortical actin bundles when ATP is depleted. Such endoplasmic filaments have not been detected in association with mitochondria or microbodies and they have not been found in the stationary cortex. These filaments are extracted from the perfused cell by ATP unless motility-inhibiting levels of cytochalasin B are present. The filaments are not detectable in cells inactivated in solutions containing high (10(-4) M) Ca++ concentrations even when the Ca++ level is subsequently lowered. Consistent with their being required for motility, cytoplasmic streaming cannot be effeiciently reactivated by ATP in such filament-depleted cells. The possibility is discussed that the filaments contain myosin and that the endoplasmic reticulum with which they are associated has a major role in generating and transmitting the motive force for streaming.