I-R hybrid dysgenesis in D. melanogaster is controlled by transposable elements known as I factors. We have determined the base sequences of one complete I factor and the ends of six others. The ends of these elements are highly conserved and are flanked by target site duplications varying in length from 10-14 bp. There are no terminal repeats, and the 3' end of one strand is A-rich, having 4-7 tandem repeats of the sequence TAA. This sequence organization is similar to that of mammalian LINEs, or L1 elements. The complete I factor sequence contains two long open reading frames, ORF1 and ORF2, of 1278 and 3258 bp. ORF1 encodes a possible nucleic acid-binding protein, and part of the amino acid sequence of ORF2 is similar to that of viral reverse transcriptases and polypeptides encoded by L1 elements. These results suggest that I factors transpose by reverse transcription of a full-length RNA.