Six semispecies constitute the Drosophilia paulistorum complex. In crosses between them, male progeny are sterile and the fertile hybrid females continue to produce sterile sons. Intracellular symbionts are found in all D. paulistorum flies. Apparently these endosymbionts are the streptococcal L-forms isolated from the semispecies. These L-forms display the same host-specific infertility exhibited in the hybrid state and a semispecies-specific male sterility pattern can be demonstrated by injection of these L-forms. The major question is: How has the superspecies D. paulistorum co-evolved with its omnipresent symbionts? Using host D. paulistorum semispecies and L-forms isolated from them, specific objectives concern: a) tracing the mechanism by which introduced isolates obtained from alternate semispecies cause male sterility; and b) clucidating the microbial bases of this host- symbiont recognition and regulation. A variety of approaches and techniques will be employed, as time permits, to plot the course of L-form infection, these include insect microinjection, establishment of aposymbiontic strains, and possible extensions of host ranges to survey for potential insecticide usage.