Axonemal dyneins have two or three globular heads joined by flexible tails to a common base, with each head/tail unit consisting of a single heavy-chain polypeptide of relative molecular mass greater than 400,000. The sizes of the components have been deduced by electron microscopy. The isolated beta heavy chain of sea urchin sperm flagella, which is immunologically identical to that of the embryo cilia, is of particular interest as it retains the capability for microtubule translocation in vitro. Limited proteolysis of the beta heavy chain divides it into two fragments, A and B, which sediment separately at 12S and 6S, and possibly correspond to the head and tail domains of the molecule. Dynein ATPase is the energy-transducing enzyme that generates the sliding movement between tubules that underlies the beating of cilia and flagella of eukaryotes, and possibly also other large intracellular movements. Here we report that the deduced amino-acid sequence of the beta heavy chain of axonemal dynein from embryos of the sea urchin Tripneustes gratilla has 4,466 residues and contains the consensus motifs for five nucleotide-binding sites. The probable hydrolytic ATP-binding site can be identified by its location close to or at the V1 site of vanadate-mediated photo-cleavage. The general features of the map of photocleavage and proteolytic peptides reported earlier have been confirmed, except that the map's polarity is reversed. The predicted secondary structure of the beta heavy chain consists of an alpha/beta-type pattern along its whole length. The two longest regions of potential alpha helix, with unbroken heptad hydrophobic repeats 120 and 50 amino acids long, may be of functional importance. But dynein does not seem to contain an extended coiled-coil tail domain.