Previous experiments discovered high rates of chromosomal rearrangements in the progeny of males containing a telomeric segment of Drosophila koepferae in a D. buzzatii genetic background (segmental males). We have performed similar experiments, designed to test whether this chromosomal instability could be explained by a phenomenon similar to P-M hybrid dysgenesis or, alternatively, by a generalized telomeric effect. However, the results obtained have not allowed us to fully characterize this process, because we have not observed chromosomal rearrangements in the progeny of the putative unstable males. Our results suggest that chromosomal instability is independent of the introgressed fragment. A reasonable hypothesis to explain these results is that mutator factors are occasionally introduced by the hybridization. The effect of sampling, caused by the fact that only a small region is introgressed in a particular line, may explain why only some hybridizations lead to instability.