Crosses between the Drosophila melanogaster y2sc1waG strain or some of its derivatives and the FM4 strain yielded insertional mutagenesis with a frequency of 10(-3)-10(-4). The system differs in several respects from the known cases of hybrid dysgenesis: (i) it does not depend on the direction of a cross; (ii) destabilization continues for a long time after initial crosses; (iii) mutations may occur at different stages of development. The mutation in the yellow locus has been cloned and found to depend on insertion into the coding region of the gene of a novel mobile genetic element designated as Stalker. The sequencing of Stalker termini reveals 405 bp direct repeats (LTRs) and a target 3 bp duplication, as well as some other sequences typical of retrovirus-like retrotransposons. The number of Stalker copies per genome and chromosomal localization vary among D. melanogaster strains. Before crosses, the location of Stalker on chromosomes is fairly stable in a particular strain but thereafter numerous changes in Stalker distribution take place. Most novel substrains are internally heterogenous which is indicative of the continuing Stalker transposition. Other mobile elements tested do not move. Possibly, only Stalker is mobilized in the system. Many known and novel mutations have been obtained. Comparison of their genetic localization with Stalker distribution suggests that the majority of them have been induced by the Stalker insertion.