A P-element mobility excision assay was used to determine if non-drosophilid insects could support P gene vector function. Present studies included the testing of Muscids, Sphaerocerids, and Phorids, none of which were able to support P mobility. A new excision indicator plasmid was developed allowing the detection and recovery of virtually all P-element excision products. The frequency and sequence analysis of excision products from Drosophila melanogaster and another drosophilid, Chymomyza procnemis, indicated both quantitative and qualitative differences in the activity of transposase. The quantitative relationships observed in the original assay were maintained, and qualitative differences in transposase activity were reflected in the sequence of the empty donor sites. The results suggest that host factors are involved in cutting and ligating P-element DNA during excision, with transposase facilitating these processes. Possible limitations on P mobility by abnormal transposase transcript processing were tested in Anastrepha suspensa using transposase-encoding plasmids having deleted intron sequences. A transposase cDNA supported normal P excision in D. melanogaster, and a low level of mobility in A. suspensa. Possible applications of gene transfer in insects are presented, in particular methods to genetically sterilize and sex insects for the sterile-insect technique.