Mobile genetic elements are found in the genomes of many organisms, and because of their effects on genes and their ability to induce chromosomal rearrangements they are an important source of genetic variability. Transposition rates are usually found to be low, estimated at around 10(-3) per generation. Higher rates of transposition are observed, however, in crosses between certain strains of Drosophila melanogaster ('hybrid dysgenesis'), which can lead to a dramatic rearrangement of many mobile elements ('transposition bursts'). We have studied the chromosomal distribution of mdg-1 and copia mobile elements in 17 highly inbred lines of D. melanogaster, after 69 generations of sib-mating. Most lines show no changes, but one showed a complete reshuffling of the copia element. We conclude that the transpositions of the copia element in this line occurred rapidly in a few generations. This phenomenon, distinct from 'transposition bursts' in that only copia elements are involved, may account for the instability sometimes observed in inbred lines and may be important in creating genetic variability in highly homozygous populations.