The initial luminescent response to photic stimulation of dark-maintained specimens of the midwater shrimp, Sergestes similis Hansen, differed from the conventional counterillumination response. Animals were initially unresponsive to light; bioluminescence was only induced after a latency of 3 min. Maximum intensity was reached after approximately 25 min. During the induction process, light emission from the anterior light organs was frequently observed prior to output from the posterior organ. Once luminescence was induced, responses exhibited the typical fast kinetics of the counterillumination response and changes in light organ output occurred synchronously. Visual input was necessary to maintain this state. Dark readaptation of counterilluminating animals resulted in a return to the slow response kinetics characteristic of untested animals. Because eyestalk ablation or crushing caused immediate production of luminescence in previously untested animals, the slow induction did not involve the ability of the light organs to produce light. Serotonin was effective in stimulating bioluminescence in intact animals; the induction of light emission proceeded at a rate similar to that for photic stimulation. Other putative neurotransmitters, including norepinephrine, acetylcholine, GABA, and L-glutamic acid, did not stimulate bioluminescence. Isolated light organs exhibited high background levels of light emission, which were unchanged by serotonin treatment. However, serotonin was effective in stimulating luminescence in animals with ablated eyestalks. These results suggest a dual control system involved in the induction and maintenance of bioluminescence in S. similis.