Cyclic ADP-ribose (cADPR)-induced Ca²⁺ release which was similar in mechanism to Ca²⁺-induced Ca²⁺ release (CICR) through ryanodine receptor (RyR) was confirmed with unfertilized eggs of Hemicentrotus pulcherrimus sea urchins. We cloned cDNAs for sea urchin egg RyR (suRyR) which encode a protein of 5,317 amino acids with molecular mass of 597 kDa. suRyR shares common structural features with known RyRs: the well conserved C-terminal domain which forms a functional Ca²⁺ channel, and a large hydrophilic N-terminal domain. Phylogenetic analysis indicates that suRyR branched from three isoforms of vertebrates before they were diverged, suggesting that it might be encoded by a single gene. Four in-frame insertions were found in suRyR cDNAs, one of which was novel and unique in having a cluster of serine residues. The transcripts with and without these insertions were all found in the egg RNA. These results suggest that suRyR with several alternative spliced variants may be expressed as functional CICR channels in sea urchin eggs, which might also be involved in cADPR-induced Ca²⁺ release.