The alpha beta dimer and the gamma subunit of the Chlamydomonas outer arm dynein were solubilized by treating isolated axonemes with 0.6 M KCI, and purified by sucrose density gradient centrifugation. The axonemes were from an ida1 mutant to eliminate contamination of outer arm subunits by inner arm dynein 11, and the axonemes were pre-extracted with 0.6 M CH3COOK to remove non-dynein protein that might otherwise contaminate outer arm dynein fractions in the sucrose gradient. In addition, purer fractions of outer arm dynein subunits were obtained by modifying the centrifugation conditions to take advantage of the propensity of the dynein to dissociate under high hydrostatic pressure in the presence of Mg2+. When sucrose gradient fractions containing the gamma subunit were added to a fraction containing the purified alpha beta dimer under conditions expected to promote reassociation of the subunits to form a trimeric outer arm dynein complex [Takada et al., 1992: J. Biochem, 111:758-762], the total ATPase activity of the mixture was suppressed to a level lower than that of the original alpha beta dimer fraction. The inhibition paralleled the distribution of gamma subunit in the sucrose gradient, was saturable, and was maximum at an approximately equimolar ratio of the gamma subunit to the alpha beta dimer. These results indicate that when the gamma subunit interacts with the alpha beta dimer, the latter's ATPase activity is modulated downward. Previous results showed that interaction of the alpha subunit with the beta subunit suppressed the beta subunit's ATPase activity [Pfister and Witman, 1984: J. Biol. Chem. 259:12072-12080]. Thus, the total ATPase activity of the outer arm dynein is dependent upon communication between all three subunits within the arm.