We have measured the transposition frequencies of six IS1-flanked transposable elements ranging in size between 2.6 and 10.5 kb. The frequency of transposition decreases approximately exponentially with increasing length by two orders of magnitude over this size range, and is reduced by about half for each additional 1 kb of DNA. The simplest explanation for these results is that transposition occurs by a processive mechanism initiating at one end of the element and proceeding to the other extremity, with a constant probability of abortion per unit length of DNA.