A new allele of the Chlamydomonas oda4 flagellar mutant (oda4-s7) possessing abnormal outer dynein arms was isolated. Unlike the previously described oda4 axoneme lacking all three (α, β, and γ) outer-arm dynein heavy chains, the oda4-s7 axoneme contains the α and γ heavy chains and a novel peptide with a molecular mass of ~160 kD. The peptide reacts with a mAb (18βB) that recognizes an epitope on the NH2-terminal part of the β heavy chain. These observations indicate that this mutant has a truncated β heavy chain, and that the NH2-terminal part of the β heavy chain is important for the stable assembly of the outer arms. In averaged electron microscopic images of outer arms from cross sections of axonemes, the mutant outer arm lacks its mid-portion, producing a forked appearance. Together with our previous finding that the mutant oda11 lacks the α heavy chain and the outermost portion of the arm (Sakakibara, H., D. R. Mitchell, and R. Kamiya. 1991. J. Cell Biol. 113:615-622), this result defines the approximate locations of the three outer arm heavy chains in the axonemal cross section. The swimming velocity of oda4-s7 is 65 ± 8 μm/s, close to that of oda4 which lacks the entire outer arm (62 ± 8 μm/s) but significantly lower than the velocities of wild type (194 ± 23 μm/s) and oda11 (119 ± 17 μm/s). Thus, the lack of the β heavy chain impairs outer-arm function more seriously than does the lack of the α heavy chain, suggesting that the α and β heavy chains play different roles in outer arm function.