Two structures on the distal ends of Chlamydomonas flagellar microtubules are described. One of these, the central microbutule cap, attaches the distal ends of the central pair microtubules to the tip of the flagellar membrane. In addition, filaments, called distal filaments, are observed attached to the ends of the A-tubules of the outer doublet microtubules. Inasmuch as earlier studies suggested that flagellar elongation in vivo occurs principally by the distal addition of sublnits and because it has been shown that brain tubulin assembles in vitro primarily onto the distal ends of both central and outer doublet microtubules, the presence of the cap and distal filaments was quantitated during flagellar resorption and elongation. The results showed that the cap remains attached to the central microtubules throughout flagellar resorption and elongation. The cap was also found to block the in vitro assembly of neurotubules onto the distal ends of the central microtubules. Conversely, the distal filaments apparently do not block the assembly of neurotubules onto the ends of the outer doublets. During flagellar elongation, the distal ends of the outer doublets are often found to form sheets of protofilaments similar to those observed on the elongating ends of neurotubules being assembled in vitro. These results suggest that the outer doublet microtubules elongate by the distal addition of subunits, whereas the two central microtubules assemble by the addition of subunits to the proximal ends.