Twenty four or 48 hr immersion of Anopheles balabacensis in S-31183 (2-[1-methyl-2-(4-phenoxyphenoxy)ethoxy]pyridine) caused higher lethal effect, mostly interfering with pupation, as the age of the last instar larvae at the time of treatment increased. The immersion of newly pupated animals also brought rather high mortality. Even immersion in 0.001 ppb of S-31183 of the larvae shortly before pupation greatly retarded the pupation development, and immersion in 0.04 ppb for 48 hr killed half of the treated animals. Further, it was found that the adult mosquitoes, which survived 48 hr of immersion in 0.005 ppb of S-31183 during their last larval instar, showed considerabel reduction in sperm and egg production, and also in blood feeding and copulating activity.