A set of insertion elements has been found in the ribosomal DNA (rDNA) internal transcribed spacers in four Orders of sharks (Carcharhiniformes, Hexanchiformes, Lamniformes, and Orectolobiformes). These elements are from 303 to 575 nucleotides in length, and show all the RNA folding characteristics of group I introns, with each element having nine to eleven pairing regions and a conserved core region. Each of these putative introns has been cloned into RNA expression vectors and in vitro splicing experiments are underway. If these elements are capable of in vitro splicing, this will be the only group I intron within rDNA reported for any metazoan. Also, this would be the only report of a group I intron within a transcribed spacer region. If these elements are incapable of in vitro splicing, they may represent remnants of a very ancient insertional event by a group I intron ancestor. Aside from the biological and evolutionary importance of these elements, their sequences vary greatly and so have the potential for use as species-specific markers for shark identification.