1. The physical appearance and bioluminescence behavior, and light-emitting reaction of the marine ostracod crustacean, Cypridina serrata, are described. 2. In the natural environment of the sea, the free-swimming C. serrata appears to emit almost instantaneously a bright blue luminous cloud when stimulated with artificial light. 3. The method of light production, consisting of the ejection of luciferin and luciferase into sea water, and the color of light are similar to that of C. hilgendorfii. 4. In captivity, C. serrata emits apparent spontaneous flashes of light, whose duration is approximately 1.5 seconds, with an apparent latency of 500-800 milliseconds. 5. C. serrata luciferase cannot be distinguished from C. hilgendorfii luciferase by gel elution chromatography but may be distinguished immunochemically. 6. The luminescence of C. serrata is due to a first order reaction, similar to that of C. hilgendorfii. The luciferins and luciferases of both organisms cross-react to give light. 7. The luminescence of C. serrata, like C. hilgendorfii, is oxygen dependent. 8. C. serrata luciferin is similar, if not identical, to C. hilgendorfii luciferin when compared by paper chromatography.