Mobilization of the P family of transposable elements in Drosophila melanogaster occurs in the hybrid progeny of males from an element-bearing strain (P strain) and females from an element-free strain (M strain). We tested whether the same crosses could mobilize other families of transposable elements. A mating scheme was used in which a set of X chromosomes was kept for 20 generations in either the active condition (known as hybrid dysgenesis) or the inactive condition (nondysgenic). Examination of 19 families of transposable elements by in situ hybridization indicated that only the P family was measurably mobilized under dysgenic conditions. Thus, P-M hybrid dysgenesis does not increase the transpositional activity of other families of transposable elements in D. melanogaster. We discuss possible explanations for several published reports to the contrary.