Oplophorus luciferin is shown to be 8-benzyl-2- (p-hydroxybenzyl)-6-(p-hydroxyphenyl)imidazo [1,2-a] pyrazin-3(7H)-one. MANY species of bioluminescent shrimps are known among the euphausids and decapods. Two species of deep-sea decapods, Systellaspis debilis and Oplophorus gracilorostris, have been shown to give a luciferin-luciferase (GL) reaction to produce in vitro bio1urninescence.1 Johnson et al.' extracted crude luciferin from the latter shrimp. Later, Yamaguchi proposed structure (I) for this luciferin without conclusive evidence. We have isolated pure luciferin from each of the title shrimps and identified it as compound (11). This is the first report that proves (11) to be a true luciferin, the substrate for an L-L reaction, although there have been several reports concerning compound (11). Thus, (11) was assumed to exist in a modified form in the Aequorea photoprotein, aeq~orin;3 it was isolated from liver of the squid,Watasenia scintill~ns,4 and considered as a possible precursor of Watasenia luciferin (111);5 Renilla luciferin, which was isolated as its sulphate from Renilla reniformis, could have the same structure as (11),3 although Hori et al. assigned structure (IV) to it;6 and Cavernularia luciferin was also assumed to be (11).7 Interestingly, the luciferin from the crustacean decapods is closely related to that of coelenterates rather than that of crustacean Cypridina.