Many full-length copies of a long interspersed repetitive element family, designated Juan-C, are reiterated in the genome of Culex pipiens mosquitoes. The complete Juan-C elements have a length of 4.48 kb. They are terminated at one end with an adenosine-rich sequence preceded with an AATAAA polyadenylation signal, lack terminal repeats and cause duplication of the host DNA at the site of their integration. Full-length Juan-C copies display two long open reading frames potentially encoding two proteins. The first one includes a domain typical of nucleic-acid-binding proteins, while the second resembles reverse transcriptases. Therefore, Juan-C elements are similar to LINE retroposons in their overall genetic organization and can probably be transposed by reverse transcription of an RNA intermediate. Juan-C elements are most similar in their sequence and coding potential to the Juan-A elements which are reiterated in mosquito species belonging to the genus Aedes. They also display homologies with some Drosophila LINEs such as Jockey, suggesting that all these elements have arisen from a common precursor. Nearly identical full-length Juan-C copies are amplified in C. pipiens strains from different continents. This finding that Juan-C retroposons reiterated in different strains form an homogeneous family is interpreted to indicate that these elements have spread recently in the C. pipiens species.