The syllid polychaete Odontosyllis phosphorea produces brilliant displays of green bioluminescence during mating swarms. We studied freshly collected individuals of O. phosphorea in the laboratory to understand the characteristics of its luminescent system. Light emission appeared as an intense glow after stimulation with potassium chloride, and was associated with secreted mucus. The mucus was viscous, blue in color, and exhibited a long-lasting glow that was greatly intensified by addition of peroxidase or ammonium persulfate. The emission spectrum of mucus-associated bioluminescence was unimodal, with a maximum emission in the green spectrum between 494 and 504 nm. The fluorescence emission spectrum was similar, but the fluorescence intensity was low unless it originated from mucus that had already produced light, suggesting that the oxidized product of the light production is the source of fluorescence. Individuals as small as 0.5–1.0 mm produced bioluminescence that was mainly internal and not secreted as mucus. The early occurrence of bioluminescence in the life cycle of members of O. phosphorea suggests that bioluminescence may be used for purposes other than attracting mates. The luminous system was functional at temperatures as low as -20°C and was degraded above 40°C. Mixing hot and cold extracts of the mucus did not result in reconstituting original levels of light emission. Additionally, mucus samples exposed to oxygen depletion by bubbling with argon or nitrogen were still able to produce intense bioluminescence. These results suggest that bioluminescence from the mucus may involve a photoprotein rather than a luciferin–luciferase reaction.