DmTTF is a Drosophila mitochondrial DNA-binding protein, which recognizes two sequences placed at the boundary of clusters of genes transcribed in opposite directions. To obtain in vivo evidences on the role of DmTTF, we characterized a DmTTF knock-down phenotype obtained by means of RNA interference in D.Mel-2 cells. By a combination of RNase protection and real-time RT-PCR experiments we found that knock-down determines remarkable changes in mitochondrial transcription. In particular, protein depletion increases not only the level of (+) and (-)strand RNAs mapping immediately after of the two protein-binding site, but also that of transcripts located further downstream. Unexpectedly, depletion of the protein also causes the decrease in the content of those transcripts mapping upstream of the protein target sites, including the two rRNAs. The changes in transcript level do not depend on a variation in mitochondrial DNA (mtDNA) content, since mtDNA copy number is unaffected by DmTTF depletion. This work shows conclusively that DmTTF arrests in vivo the progression of the mitochondrial RNA polymerase; this is the first ever-obtained evidence for an in vivo role of an animal mitochondrial transcription termination factor. In addition, the reported data provide interesting insights into the involvement of DmTTF in transcription initiation in Drosophila mitochondria.