The P-element-mediated gene transfer system was used to introduce Drosophila teissieri 5S genes into the Drosophila melanogaster genome. Eight transformed D. melanogaster strains that carry D. teissieri 5S mini-clusters consisting of 9-21 adjacent 5S units were characterized. No genetic exchanges between D. melanogaster and D. teissieri 5S clusters were detected over a 2-year survey of the eight strains. The occurrence of small rearrangements within the D. melanogaster 5S cluster was demonstrated in one of the transformed strains.