Aequorin is a bioluminescent protein which consists of a polypeptide chain (apoaequorin), coelenterate luciferin, and bound oxygen. Aequorin produces blue light upon binding Ca²⁺. We have isolated six recombinant pBR322 plasmids which contain apoaequorin cDNA sequences. A mixed synthetic oligonucleotide probe was used to identify these cDNAs. An extract of an E. coli strain possessing the largest cDNA contained apoaequorin. This apoaequorin can be converted to aequorin in the presence of coelenterate luciferin, 2-mercaptoethanol, and O₂. This cDNA is therefore apparently full-length.