CARBON dioxide has been used as a narcotic for invertebrate animals for many years ; but the nature of its action is not yet understood. It has recently come into prominence in the discovery of strains of Drosophila sensitive and resistant to carbon dioxide (reviewed by l'Heritier1). In using carbon dioxide as a means of temporarily immobilizing aquatic invertebrates for extraction of blood samples, we have been struck by some remarkable interspecific differences in sensitivity and have been led to investigate the problem.