We have used computer averaging of electron micrographs from longitudinal and cross-sections of wild-type and mutant axonemes to determine the arrangement of the inner dynein arms in Chlamydomonas reinhardtii. Based on biochemical and morphological data, the inner arms have previously been described as consisting of three distinct subspecies, I1, I2, and I3. Our longitudinal averages revealed 10 distinguishable lobes of density per 96-nm repeating unit in the inner row of dynein arms. These lobes occurred predominantly but not exclusively in two parallel rows. We have analyzed mutant strains that are missing I1 and I2 subspecies. Cross-sectional averages of pf9 axonemes, which are missing the I1 subspecies, showed a loss of density in both the inner and outer portions of the inner arm. Averages from longitudinal images showed that three distinct lobes were missing from a single region; two of the lobes were near the outer arms but one was more inward. Serial 24-nm crosssections of pf9 axonemes showed a complete gap at the proximal end of the repeating unit, confirming that the I1 subunit spans both inner and outer portions of the inner arm region. Examination of pf23 axonemes, which are missing both I1 and I2 subspecies, showed an additional loss almost exclusively in the inner portion of the inner arm. In longitudinal view, this additional loss occurred in three separate locations and consisted of three inwardly placed lobes, one adjacent to each of the two radial spokes and the third at the distal end of the repeating unit. These same lobes were absent in ida4 axonemes, which lack only the I2 subspecies. The I2 subspecies thus does not consist of a single dynein arm subunit in the middle of the repeating unit. The radial spoke suppressor mutation, pf2, is missing four polypeptides of previously unknown location. Averages of these axonemes were missing a portion of the structures remaining in pf23 axonemes. This result suggests that polypeptides of the radial spoke control system are close to the inner dynein arms.