SummaryDestabilization in somatic cells of P-element insertions in the X-linkedsingedgene ofDrosophila melanogasterhas been studied. We have shown that some but not all unstable P-element insertions insingedcan form mosaics. The cause of this variation is not clear from studies of the restriction maps of the mutations tested. The transposable element movements occur early in development and require, in addition to an appropriate P-element insertion insinged, atrans-acting maternal effect component. Movements appear to occur preferentially in attached-X stocks. However, the maternal effect component maps to the central region of chromosome 2.