The 18 S dynein of the outer arm of Chlamydomonas flagella contains two different heavy polypeptide chains (Mr approximately equal to 340,000), two intermediate chains (Mr = 69,000 and 78,000), and eight light chains (Mr = 8,000-20,000). We report here that when purified 18 S dynein is dialyzed against a low ionic strength solution, it is dissociated into two smaller subunits which can then be separated and purified by sucrose density gradient centrifugation. One subunit contains one of the heavy chains and a Mr = 16,000 light chain; the other contains the other heavy chain and the remaining intermediate and light chains. Both subunits have ATPase activity. When recombined in the presence of 5-25 mM KCl, the subunits reassemble to form a particle similar to native 18 S dynein; neither subunit by itself can reform such a particle. 18 S dynein is therefore a heteropolymer containing two compositionally distinct subunits. Because the complete outer arm contains both 12 S and 18 S dyneins, the arm must have a total of three sites of ATP binding and hydrolysis: one associated with 12 S dynein and two with 18 S dynein.