Bioluminescence o f euphausiids takes place when a fluorescent tetrapyrrole F and a highly unstable protein P react in the presence o f oxygen. A previous study on the euphausiid Meganyctiphanes norvegica indicated that F acts as a catalyst and P is consumed in the luminescence reaction, differing f r o m the luminescence system of dinoflagellates in which a tetrapyrrole luciferin, nearly identical to F, is enzymatically oxidized in the presence o f dinoflagellate luciferase. In the present study, P was extracted from Euphausia pacifica as well as f r o m M. norvegica, then purified separately by affinity chromatography on a column of biliverdin-Sepharose 48, completing the whole process in less than 5 h. The samples of P obtained from both species had a molecular weight o f 600,000, a purity o f about 80%. and a specific activity 50-100 times greater than that previously found. The activity o f P rapidly decreased in solutions, even at 0Β°C. and the inactivation o f P derived from M. norvegica was more than four times faster than that derived from E. pacifica. The kinetics o f the luminescence reaction was investigated with F and P whose concentrations were systematically varied. The reaction was characteristically slow and involved t w o different reaction rates; the turnover number a t 0Β°C was 30/h for the initial 20min and 20/h after the initial 1 h. The total light emitted in a 50-h period indicated that the bioluminescence quantum yield o f F was about 0.6 at 0Β°C. and P recycled many times in the luminescence reaction. Thus, the present results conclusively show that F is a luciferin and P is a luciferase o f an unusually slow-working type, contrary t o early report.