A family of short interspersed repetitive elements (SINEs), designated mermaid, is present in the genomes of fish, amphibian and primates, but absent in the mouse genome. We have demonstrated that the sequences of the mermaid family are highly polymorphic in the zebrafish genome as in the human genome. We have also shown that the mermaid sequence can be used to recover zebrafish specific DNA from zebrafish-mouse cell hybrids by using mermaid-specific oligonucleotides as PCR primers. Thus, the mermaid family serves as a valuable genetic tool for the zebrafish genome mapping.