Microbes impact the development and physiology of animals. Quorum sensing describes a signaling mechanism that coordinates behaviors among bacterial cells. How quorum sensing functions within a host remains poorly understood. Vibrio fischeri is a bacterium that colonizes the light organ of the Hawaiian bobtail squid, where the resulting populations produce bioluminescence via quorum sensing. I found that separate populations of dark V. fischeri mutants communicate by quorum sensing within the host to produce luminescence.