With the use of standard dissection, electron microscopic and histochemical fluorescence techniques branchial nerve fibers have been traced from the visceral ganglion to the ciliated epithelium of the gill in the mussel, Mytilus edulis. Hitherto underscribed nerve fibers have been shown to lie adjacent to gill filaments and to penetrate the fibrous basal lamina under the gill epithelium. Nerve fibers have been observed subjacent to postlateral. ciliated lateral and frontal cells. Homologous lateral ciliated cells have been demonstrated to be connected by septate junctions. The localization of 5-HT in the branchial nerve within the gill epithelium has been studied by means of histochemical fluorescence. Severing the branchial nerve or intramuscular injections of reserpine depleted the 5-HT in the gill. Intramuscular injections of nialamide resulted in an increase of specific fluorscence.