Poseidon and Neptune are two ancient lineages of retroposons related to the Penelope element from Drosophila virilis. They have been identified in various teleost fish species, including the medakafish (Oryzias latipes), and the pufferfishes Fugu rubripes and Tetraodon nigroviridis, whose genomes are currently being sequenced. Some of these elements are highly reiterated in fish genomes. Penelope-related elements were also identified in blood fluke, shrimp, sea urchin, cichlid fish and frog, showing that they are widespread in animals. Penelope-related retroposons were not detected among sequences from the Drosophila melanogaster and human genome projects, suggesting that they have been lost from certain animal lineages. A sequence encoding a putative Uri (also called GIY-YIG) endonuclease domain was detected downstream from the gene for reverse transcriptase. To the best of our knowledge, this type of endonuclease sequence has previously been identified in group I introns and in genes for prokaryotic excinucleases but not in retrotransposable elements. Penelope-related elements are frequently truncated at their 5' ends and can also be flanked by long terminal repeat-like structures. Phylogenetic analysis of the reverse transcriptase domain failed to assign Penelope-related retroposons to one of the major groups of retroelements. Overall, therefore, the evidence strongly suggests that these sequences represent a new group of retrotransposable elements.