We have previously shown that the activity of functional I retrotransposons (I factors) introduced into Drosophila devoid of such elements can be repressed by transgenes containing an internal fragment of the I factor itself and that this repressing effect presents the characteristic features of homology-dependent gene silencing or cosuppression. Here we show that the same transgenes can induce silencing of a nonhomologous reporter gene containing as the sole I-factor sequence its 100-bp promoter fragment. Silencing of the nonhomologous reporter gene shows strong similarities to I-factor cosuppression: It does not require any translation product from the regulating transgenes, sense and antisense constructs are equally potent, and the silencing effect is only maternally transmitted and fully reversible. A search for genomic I-like sequences containing domains with similarities to those of both the regulating and the reporter transgenes led to the identification of four such elements, which therefore could act as intermediates-or relays-in the cosuppression machinery. These results strongly suggest that ancestral transposition-defective I-related elements, which are naturally present in the Drosophila genome, may participate per se in the natural conditions of I-factor silencing.