The majority of TN10 insertions in the lacZ gene of Escherichia coli occurred in a small region of the promoter distal part of the gene. The resulting mutations were polar on lacY and reverted to Lac+ at a frequency of 10(-8). None of the revertants were Tcr. Furthermore Lac+ Tcr revertants could not be selected directly. Relief of polarity revertants of the lacZ::Tn10 mutants were formed at a frequency of 10(-5) - 10(-4). Most resulted from a deletion event internal to the transposon which removed the Tcr genes and the putative transcription terminator. It is postulated that a fragment of Tn10 remains at the original insertion point to cause a revertible Lac- mutation.