Background: C. elegans BAG neurons respond to environmental CO2. Results: By isolating BAG neurons in culture, we show that they detect CO 2 independently of intracellular or extracellular acidosis or bicarbonate. Conclusion: C. elegans BAG neurons detect molecular CO2. Significance: Cells can directly detect the respiratory gas CO2 using dedicated receptors. Similar mechanisms might mediate some of the effects of CO2 on other physiological systems. © 2013 by The American Society for Biochemistry and Molecular Biology, Inc.