Molecular analysis of hyperunstable mutations in the adjacent yellow and scute loci was performed in a Drosophila melanogaster strain y2ns scme obtained earlier. The y2ns scme combination originated from the original y+ns mutation associated with the insertion into the regulatory region of the yellow gene of a chimeric sequence flanked by deletion copies of the P element. The new combination of mutations resulted from an inversion between P-element copies located in the adjacent yellow and scute loci. The inversion was flanked by two P-element copies at one side and one copy at the other side. All three P-element copies had various internal deletions and, therefore, could not code for active transposase. An introduction of the delta (2-3) gene for transposase induced mutagenesis in the y2ns scme strain. Most mutations observed were associated with either excision of a single P-element copy or reinversion resulting from the recombination between the ends of P elements located in the yellow and scute genes. Genetic analysis showed that the alleles resulting from the recombination at a high frequency reverted to the original state.