A family of long interspersed repetitive elements (LINEs) dispersed in the genome of Aedes mosquitoes is described. Basically, full-length copies of the element designated Juan-A are dispersed in the genome of A. aegypti, but some elements are truncated or deleted. Complete Juan-A elements are 4.7 kb long, and their overall genetic organization is similar to that of LINEs from other species in which this class of nonviral retrotransposons has been described. Juan-A elements are terminated at the 3' end by an adenosine(A)-rich sequence and are flanked by target-site duplications. They display two long open reading frames potentially encoding two polypeptides. The first one contains Cys-rich motifs typical of nucleic-acid-binding proteins, while the other shows homology to the reverse transcriptases. These features are characteristic of LINE retroposons and indicate that Juan-A elements can be transposed by reverse transcription of an RNA intermediate. Furthermore, Juan-A retroposons display significant homologies with the Drosophila LINEs Jockey and F, suggesting that all these elements have arisen from a common precursor. The full-length Juan-A copies which are amplified in the genomes of various strains belonging to the three non-sibling species, A. aegypti, A. albopictus and A. polynesiensis, form an internally homogeneous family. These data are interpreted to indicate that active Juan-A retroposons underwent a recent amplification in the strains analyzed. Furthermore, they suggest that these elements have spread by horizontal transfer between the three non-sibling species.