Unfertilized eggs of sea urchins (Hemicentrotus pulcherrimus) demonstrated cyclic ADP-ribose (cADPR)-induced Ca²⁺ release and caffeine-induced Ca²⁺ release, both of which were considered to be mediated through the ryanodine receptor (RyR). We cloned cDNAs for sea urchin egg RyR (suRyR), which encode a 597-kDa protein of 5,317 amino acids. suRyR shares common structural features with known RyRs: the well-conserved COOH-terminal domain, which forms a functional Ca²⁺ channel, and a large hydrophilic NH₂-terminal domain. suRyR shows amino acid sequence identity (43-45%) similar to the three mammalian RyR isoforms. Phylogenetic analysis indicates that suRyR branched from three isoforms of vertebrates before they diverged, suggesting that suRyR may be the only RyR isoform in the sea urchin. Four in-frame insertions were found in suRyR cDNAs, one of which was novel and unique, in that it had a cluster of serine residues. The transcripts with and without these insertions were found in the egg RNA. These results suggest that suRyR may be expressed as a functional Ca²⁺-induced Ca²⁺ release channel, which might also be involved in cADPR-induced Ca²⁺ release.