Inhibitors of NF-κB (IκBs) have been implicated as major components of the Rel/NF-κB signaling pathway, which is an important mediator of immune responses throughout much of the animal kingdom. Here we report the first characterization of a bivalve mollusc cDNA that encodes a novel IκB member called Cg-IκB showing the conservation of most IκB protein characteristics. Sequences as well as phylogenetic analyses reveal a high level of identity between not only Cg-IκB and other mollusc and insect IκB-like proteins but also similarities to vertebrate IκB-α and -ε isoforms. Expression analyses demonstrated that the transcript is widely expressed in all oyster tissues. This work is consistent with our previous discovery of several members of an NF-κB pathway in Crassostrea gigas and further sustains the hypothesis of a conserved scheme of immune gene regulation through most of the metazoans.