We had shown previously (3) that the E. coli argE gene could be turned-on by an IS3 element inserted in orientation II near the 5' end of the gene. Here we show that this effect is due to the presence of an outward promoter located on IS3. The exact site of insertion of IS3 was determined by DNA sequencing. Using the S1 nuclease mapping technique with in vivo transcribed RNA we located the promoter responsible for argE transcription on IS3 itself outside the region involved in the inverted repeats of this element. IS3 may therefore be considered as a mobile promoter.