Saxitoxin (STX), the cause of paralytic shellfish poisoning (PSP), is produced by several dinoflagellate genera including Alexandrium. Recently, several studies have demonstrated bacterial production of STX and its derivatives, and have hypothesized that bacteria associated with dinoflagellates may be responsible for toxin production during blooms. Previous work using HPLC technology showed butter clams (Saxidomus giganteus), located in areas of toxic dinoflagellate blooms, and preferentially accumulated STX in their siphons where it could remain active for years. It is suspected that continuous high siphon levels of STX and its derivatives may result from colonization of the siphon by toxin producing bacteria originally associated with the toxic dinoflagellates. By immunostaining butter clam tissues using an anti-STX polyclonal antibody, we found that STX was localized to the outer epithelial cells of the siphon. Staining appeared to be intracellular. With a positive identification of the cell type in which STX is located, the search for STX producing bacteria can proceed.