Actin is a protein component of the cystoskeleton and is involved in cell motility. It is believed generally that actin filaments are attached to the cell membrane through an interaction with membranous actin-binding proteins. By using an in vitro system composed of liposomes and actin, we have shown that actin may also interact directly with the phospholipids of the membrane. Actin deposited at the surface of the liposome is organized in two regular patterns: a paracrystalline sheet of parallel filaments in register, or a netlike organization. These interactions of actin with membrane lipids occur only in the presence of millimolar concentrations of Mg2+. These results suggest that the interaction of the cytoskeleton with the membrane involves, at least in part, a direct association of actin with phospholipids.