Field tests were conducted to assay the mating competitiveness of the genetic sexing strain (MACHO) developed for Anopheles albimanus Wiedemann. The Amayo River, which is located in the coastal plain of El Salvador, was used as the release area. Chemosterilized MACHO males, heterozygous for a Y: autosome translocation and an inversion, were released along with males and females of a newly colonized wild-type strain. The competitiveness (c) of the MACHO males averaged 0.785 for the field releases, which indicates these males are capable of inducing high levels of sterility in indigenous field populations. Also, when MACHO males (sterile and nonsterile) were compared with wild-type males and colony males in laboratory cages containing wild-type females, they were competitive. Neither the sterilization process nor the chromosomal aberrations inherent in the sexing strain impeded the mating vigor of the MACHO males.