The mobility properties of the Drosophila melanogaster P element in drosophilid and nondrosophilid species has been determined using a P-element mobility assay that is conducted transiently in insect embryos. P elements are mobilizable in all drosophilids tested, including species outside the genus Drosophila but not in the related Tephritidae (order: Diptera), although the P-element gene necessary for mobility, transposase, is transcribed. These results show that without modifications P elements will not serve as general insect gene vectors and suggest that nonconserved host-encoded factors participate in the transposition of P elements. Our methods will be generally useful for analyzing the cis- and trans-acting factors required for P-element mobility in vivo and could be used to analyze the mobility properties of other transposable elements in insects.