Sex can be a bitter conflict fought with sophisticated weapons—at least in Drosophila. Females tend to mate with multiple partners to broaden the genetic diversity of their offspring, whilst males try to avoid the sperm competition that arises from female promiscuity. The mechanisms evolved by males to reduce female remating are of particular interest because of their potential use in insect pest control. In Drosophila melanogaster, mating causes a reduction in sexual receptivity and an elevation in egg production that last several days. Although these postmating responses are known to be caused by a component of seminal fluid or by sperm itself, the molecules involved have been unclear. In the July 21 PNAS, two teams independently identify the molecule responsible for the so-called "sperm effect." The two teams used different approaches to generate a male fruit fly that did not produce sex peptide (SP), a 36-residue ejaculatory protein...