Technology was developed for mass rearing males of the genetically altered MACHO strain of Anopheles albimanus Wiedemann, which allowed for elimination of females by treating the eggs with propoxur (o-isopropoxyphenyl methylcarbamate). This made it possible to eliminate virtually all the females (potential malaria vectors) being released in the field, and also reduced the space previously devoted to larval rearing by 50%, since the females were eliminated in the egg stage. Also, the difficulties involved in separating the sexes with previous techniques were eliminated. Because there is some genetic recombination, about 0.2% of the MACHO males become susceptible and an equal number of females become resistant each generation. Thus after 6-12 months, the strain is purged to remove these contaminants. With this system an average of more than 1 million pupae per day was produced during 3 weeks of a 5-week period, and an average of 968.2 thousand per day during the entire period. The pupae produced were 99.9% males with an average adult emergence of 90%.