Questions relating to the origin and regulation of mobile genetic elements are currently of considerable interest. Since it is now possible to address more precisely issues concerning the entry, dispersion, and regulation of elements within a virgin genome, one approach that may afford a better understanding of transposable elements in general could be provided by interspecific DNA transformation. Therefore, the Tc1 transposable DNA element from Caenorhabditis elegans was chosen as a proposed invading element of the Drosophila melanogaster genome. The basis for this selection resided in the inherent structural and functional similarities, as well as sequence identities, between the Caenorhabditis element and elements innate to Drosophila (e.g., P, HB1, and Uhu). Initial investigations were carried out to define a clone carrying an intact Tc1 element. This Tc1 element was inserted into a P transposon vector and two P-Tc1-ry+ constructs, differing only in insert orientation, were identified. P element mediated germ line transfer was then used to generate a transformant that was genetically and molecularly identified as containing a single, structurally intact Tc1 element at cytological location 64C4-5 on the third chromosome. The single P[(Tc1,ry+)]SAS-B insertion was thereafter mobilized by using a P[ry+ delta 2-3] element as a transposase source, and the genetic and molecular data suggested that the insertion had been successfully reintegrated to a variety of genomic locations. On the basis of genetic and molecular analyses, the Tc1 element in the P[Tc1,ry+)] transformed stock is not highly unstable in germ line and somatic tissues.