We constructed a Tn5 derivative for potential use in transposon mutagenesis of Neisseria gonorrhoeae. It was incorporated into the chromosome apparently at random following transformation, but the insertion events were dependent on a functional RecA and independent of a functional transposase. Furthermore, in most cases there was an incomplete transposon inserted with little or no IS50 insertion sequence. These observations suggest that Tn5 transposition may not be possible in N. gonorrhoeae and that this organism may have an unexplored illegitimate recombination system.