In our earlier studies, we showed that in D. virilis, similar to D. melanogaster, there are strains of three cytotypes, namely, neutral, M-like and P-like strains, depending upon their roles in HD. In D. melanogaster strains of M-cytotype do not contain functional P-elements and produce partially sterile progeny when crossed with males from Pstrains carrying multiples copies of full-size P-elements while neutral strains do not produce significant proportion of sterile progeny when crossed either with M-like or P-like strains. In D. virilis strains named by analogy with D. melanogaster “M-like strains�? including the wild-type strain 9 used in the present study, usually contain only heterochromatic, highly diverged copies of Penelope retroelements. Furthermore, such diverged copies of Penelope are located in such strains mainly in the pericentromeric heterochromatin. These strains produce high levels of gonadal sterility and other manifestations of HD when crossed with males of strain 160, which represents the only strong P-like strain described in D. virilis so far and contains multiple copies of Penelope probably playing an important role in HD. In situ hybridization on polytene chromosomes and Southern blot analysis revealed mobilization of several unrelated TEs in the progeny of dysgenic crosses. These elements include Helena, Paris, Tv1, Telemac, Ulysses and Penelope. Among these, Ulysses which represents a typical retroelement with LTRs of 2 kb in size and two ORFs, was the first element described in D. virilis and subsequently found in several visible mutations, including white, obtained in the progeny of dysgenic crosses. Furthermore, this element was found at the breakpoints of inversions detected in the progeny of dysgenic crosses and, hence, it was implicated in the formation of aberrations never high content screening inquirer before found in D. virilis. In contrast to Ulysses, another well studied LTR-containing retroelement gypsy, previously described in D. virilis, was never found in mutations in the progeny of dysgenic crosses. It has been shown by different methods that multiple active copies of Penelope are present in strain 160, while strain 9 does not carry full-size Penelope copies in the euchromatic chromosome arms. Highly diverged and apparently ancient copies of Penelope, termed? located mostly in the heterochromatic chromocenter, were, however, detected and investigated in both strains studied. In situ hybridization with polytene chromosomes and Southern blotting analysis showed that contrary to Penelope, full-size Ulysses copies are found in all D. virilis strains studied so far, with an average of copies per strain. There is molecular and genetic evidence suggesting that the TE enelope plays an important role in D. virilis HD. The Penelope retroelement does not belong to one of the previously well studied classes of TE.