The two flagella of Chlamydomonas, although similar to each other at first glance, differ in functional properties. A clear difference exists in the beat frequency: the trans-flagellum (the one farthest from the eyespot) beats with 30-40% higher frequency than the cis-flagellum (the one nearest to the eyespot) in demembranated and reactivated cell models. This difference is considered to be influenced by outer arm dynein, because the two flagella beat at almost the same frequency in cell models of oda mutants lacking the outer dynein arm. When a sample of outer arm dynein extracted and purified from the wild-type axoneme was mixed with the cell models of an oda mutant, oda1, an almost normal number of outer dynein arms became attached to the axonemes, and the wild-type level of beat frequency was recovered on reactivation with ATP addition. The frequency imbalance, however, was not restored. Unexpectedly, when a similar experiment was performed with the cell model of another oda mutant, oda6, the addition of outer arm dynein restored the cis-trans frequency imbalance in addition to the normal number of outer arms and the higher level of reactivated motility. Among other oda mutants, oda3 yielded results similar to those with oda1, whereas oda2, oda4, and oda5 yielded results similar to those with oda6. Because the only structural difference between the two groups of oda mutants is that the oda1 and oda3 axonemes lack the outer arm attachment site on the outer doublet A-tubule while the axonemes of the other mutants retain it, these findings suggest that the attachment site for the outer dynein arm is important in determining the flagellar beat frequency. This suggests that the basal portion of the outer arm dynein is important in regulating the flagellar activity and therefore the behavior of the cell.