AbstractPholasin is the photoprotein extracted from the marine bivalve Pholas dactylus. It undergoes an oxidative chemiluminescent reaction to oxypholasin with superoxide anion, hypochlorite, peroxidases and other oxidants. Since the observed absorbance and chemiluminescent emission spectra of pholasin solutions cannot be brought about solely by the amino acids composing the protein, there has to be a chemiluminescent chromophore. However, little is known about the chemical nature of this molecule. This work seeks to identify the chemical structure of the luminescent prosthetic group of pholasin.Pholasin could not be reactivated using chromophores from the hydroid Obelia geniculata (coelenterazine) and from the ostracod shrimp Vargula (formerly Cypridina) hilgendorfi. Furthermore, the reaction product of the Vargula chromophore could not be detected in solutions containing oxypholasin. Fluorescence analysis of such a solution revealed a compound with an emission spectrum (γmax 480 nm; excitation at 320 nm), resembling the emission spectrum of the chemiluminescent reaction. This fluorescent substance was separated by gel filtration. It exhibited an apparent molecular mass of < 2000. Fluorescence masurements of extracts of partially purified pholasin suggested that a flavin moiety may be involved in pholasin luminescence.