For the first time, mariner elements were found in insect parasitic nematodes. Full-length elements were isolated from the rhabditid Heterorhabditis bacteriophora. They were 1279 bp long, flanked by two 30-bp inverted repeats, and were able to encode a putative 358-amino acid transposase. These elements were present in about 30 copies in the H. bacteriophora genome, but their distribution among closely related Heterorhabditis and Steinernema genera was patchy. DNA and encoded peptide sequences of H. bacteriophora mariners showed greater similarity to the mariner of the coleopteran Carpelimus sp. than to the mariners of the rhabditid Caenorhabditis elegans. The possibility of horizontal transfer was investigated by examination of a host for Heterorhabditis nematodes, a beetle of the Phyllophaga sp. Mariner elements were found in this insect, but they were not very similar to the H. bacteriophora elements. Finally, the H. bacteriophora mariners formed a group with those of invertebrates, suggesting vertical transmission from a common ancestor.