P elements are transposable elements found in P strain, but usually not in M strain, Drosophila melanogaster, and are responsible for the hybrid dysgenesis that occurs when male D. melanogaster of the P strain mate with females of the M strain (ref. 1 and references therein). Several P elements, which vary in length and genetic effects, have now been cloned. To investigate the evolutionary origin of P elements, we have used a cloned copy of a D. melanogaster P element to look for related sequences in the genomes of six other Drosophila species. We report here that, unlike many other transposable elements found in D. melanogaster, which seem also to be present in other Drosophila species, we have found no sequences closely enough related to P elements to be detected by DNA hybridization in any other Drosophila species. This result supports the hypothesis that P elements have recently invaded D. melanogaster by horizontal transmission.