In situ hybridization with polytene chromosomes was used to demonstrate the transposition of mobile dispersed genes (mdg)-1 and 3 following the selection of flies from low reproductive activity and viability (LA stock) for high reproductive activity, viability and fitness (LA+ and HA stocks). The inbred LA stock is continuously selected for low reproductive activity and viability and maintains at least for twenty-five generations a characteristic pattern of mdg-1 distribution in 14-15 sites. Inbred LA+ and HA stocks exhibit a changed pattern of mdg-1 locations and the number of sites reaches 21-25. Parallel and independent selection for higher viability may lead to similar characteristic changes in the localization of mdg-1. In several independent experiments we observed, within one generation, a spontaneous and saltatory growth of viability and fitness in the mass-bred LA stock. In these cases new mdg-1 and mdg-3 sites reproducibly appeared to within several bands, some of them characteristic of LA+ and HA stocks. We discuss the possible role of mdg in determining the quantitative characters of individuals and their fitness.