In experiments on conjugation in Thiobacillus versutus with the use of pTAV1-less strains as recipients, we have proved that the derivative of the wild-type T. versutus cryptic plasmid pTAV1 (107 kb) marked with Tn1721 (Tc-r) transposon demonstrates Tra(-) phenotype but can be mobilized for transfer by pSa Tra(+) broad-host-range helper plasmid at a low frequency. The possibility of chromosomal gene exchange between different auxotrophic and drug-resistant T. versutus mutants has been confirmed. The previously assumed participation of plasmid pTAV1 in the above process must be excluded because conjugal transfer of chromosomal markers can be observed even when two pTAV1-free strains are mated. Formation of some classes of transconjugants can be reasonably explained only when two-directional chromosomal DNA transfer (retrotransfer) is considered. At this stage of our studies we can not propose any hypothesis on the mechanism of chromosomal gene transfer. The possible role of the megaplasmids discovered in T. versutus in chromosome mobilization needs to be elucidated.