The prokaryotic transposable element Tn5 has been found to promote the formation of adjacent deletions. The frequency of adjacent deletion formation is much lower than that of normal transposition events. Like normal transposition, however, adjacent deletion formation requires the activity of the transposase protein. The deletions can be divided into two classes, as distinguished by their endpoints. The occurrence of one of the two deletion classes is increased when the frequency of normal transposition is reduced by the introduction of a deletion or a certain base substitution at one of the two outside ends (OEs). The nature of the base substitution at the mutant OE influences the class of deletion found adjacent to the wild-type OE, even though these two ends are about 12 kbp apart. By studying the formation of these deletions, we have gained some insight into the way in which the transposase interacts with the OEs. Our observations suggest that there is a protein-mediated interaction between the two ends, that different end base pairs are involved in different transposition-related processes, and that the adjacent deletions are the result of nonproductive attempts at transposition.