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To have a better understanding of the evolutionary history of mobile elements within the nematodes, we examined the distribution and the conservation of homologues to transposable elements from Caenorhabditis elegans (Tc1, Tc2, Tc3, Tc4, Tc5, and FB1) in 19 nematode species belonging to the class Secernentea. Our results show that Tc1 elements display a distribution restricted to the family Rhabditidae with poor conservation. The Tc2 and FB1 homologous elements have the same patchy distribution within the Rhabditidae. They were only found in Caenorhabditis and in Teratorhabditis. The Tc3 element is widely distributed among nematode species. Tc3 homologous elements are present in the majority of the Rhabditidae but also in two genera within the family Panagrolaimidae, and in Bursaphelenchus, which belongs to the order Aphelenchida. Tc4 and Tc5 homologues show the most limited distribution of all tested elements, being strictly limited to C. elegans. These data indicate that in some cases, the distribution of transposable elements in the nematode cannot be explained by strict vertical transmission. The distribution of Tc3, Tc4, and Tc5 suggests that horizontal transmission may have occurred between reproductively isolated species during their evolutionary history. [ABSTRACT FROM AUTHOR]