Field competitive mating tests were conducted in El Salvador. C.A., to assay the dispersal ability of sterile MACHO males, the genetic sexing strain developed for Anopheles albimanus Wiedemann mosquitoes. The tests were designed to study the strategy of releasing sterile males at sites 200 m apart. Chemosterilized MACHO males were released from 2 sites, and males and females from a newly colonized wild-type strain were released from a third site. Sterility of recaptured females was used to compute an average competitiveness factor (C) of 0.74 for the sterile MACHO males, which indicates that the sterile males effectively dispersed throughout the release area and that the 200-m distance between release sites allowed effective contact between sterile males and wild females.