I factors are transposable elements of Drosophila melanogaster similar to mammalian LINEs, that transpose by reverse transcription of an RNA intermediate and are responsible for the I–R system of hybrid dysgenesis. There are two categories of strains in this species: inducer, that contain about 15 I elements at the various sites on chromosomal arms, and reactive, that lack active I factors. I elements occur in various Drosophila species. Potentially functional I factors from Drosophila teissieri can transpose when introduced by P-element-mediated transformation in a reactive strain of Drosophila melanogaster. We have studied the properties of Drosophila melanogaster strains into which such an I factor from Drosophila teissieri, named Itei, was introduced. Typical hybrid dysgenesis is produced when males carrying Itei are crossed with reactive females. However, more than one copy of the element seems necessary to produce dysgenic traits, whereas only one I factor of Drosophila melanogaster seems to be sufficient. The copy number of Itei in transformed lines maintained by endogamous crosses increases rapidly and stabilizes at values similar to those observed in inducer strains. As Drosophila teissieri contains much fewer copies than the Drosophila melanogaster strains, this suggests that the copy number of I elements is not simply regulated by sequences present in the element itself.