Carbon dioxide exposure of larvae and pupae (Culex tarsalis, Cx P. quinquefasciatus, Anopheles albimanus) results in inactivity and anesthesia which permits speedy separation of pupae. Recovery was nearly complete in groups exposed for less than 8 minutes. Mortality increased with longer exposure but 45, 16 and 42% of pupae of the 3 species, respectively, survived exposures of 64 minutes.