Vibrio fischeri is a bioluminescent bacterium that enters into a symbiosis with the bobtail squid Euprymna scolopes. The bacterium colonizes a specialized light organ, in which it generates light that might help the squid to hide its silhouette from animals beneath it. Previous studies have shown that the host nitric oxide (NO) synthase is active during colonization, suggesting that V. fischeri symbionts are exposed to NO. Thus, NO might play a role in regulating the symbiosis, a role possibly analogous to that of NO in the interaction between some pathogens and their hosts. One possibility is that NO helps to exclude other species from the light organ, in which case, the response of V. fischeri to NO is of considerable interest. In this issue of Molecular Microbiology, Dunn et al. report that V. fischeri produces an NO-inducible and NO-resistant alternative oxidase (Aox) that allows respiration to continue in the presence of NO concentrations that are inhibitory to the conventional respiratory oxidases. This is an important step towards a better understanding of the role that NO plays in the Vibrio-squid symbiosis, and provides the first indication of a physiological function for a bacterial homologue of the plant Aox.