1) Chemoreception was investigated of the receptor end organ on the flank skin in several fresh water and sea water bony fish by recording the electrical responses in single fibers of the lateral-line nerve (X) and of the accessory lateral-line nerve (VII). Three species of fish, catfish (Ictalurus), mullet (Mugil) and carp (Cyprinus) were studied extensively.2) Responses of the receptor organ to monovalent cations, K+, Na+, Rb+, NH4+, Li+, and Cs+ in the solution were observed on all fishes studied. Among them K+ had the strongest effect. Inorganic anions were ineffective. In fresh water fish the sensitivity of the end organ was much superior to that of marine fish. The responses to various concentrations of Na+ were almost parallel to responses to K+, although somewhat smaller in fresh water fish, while in marine fish the sensitivity to Na+ was much inferior to that to K+. Such a fact may come from an abundance of Na+ in the environment. In those end organs divalent cations like Ca++, Mg++ and Sr++ suppressed excitatory effects of monovalent cations when the former were applied simultaneously or in sequence to the end organ. Tetrodotoxin also produced a similar effect. Such suppressive effects were reversible. After rinsing the end organ with fresh water or sea water thoroughly, those effects subsided completely.3) In euryhaline fish when they move from sea water to fresh water or vice versa, the sensitivity to K+ and Na+ changes within a certain period of time from the type of marine fish to that of fresh water fish or in the opposite direction. In a mullet the complete change of sensitivity of the end organ occurred in 7 days.4) In the case of catfish there were observed a variety of responses to chemical stimuli, not only to monovalent cations but also to divalent ones and several others as well. Some fibers were very sensitive to NH4+, or specifically to quinine or glutamate. None of them responded to sugar.5) Histological studies have disclosed that(a) the catfish has various types of end organs innervated by the lateral line nerve and the accessory lateral-line nerve on the flank skin; canal neuromasts, large and small pit organs and many terminal buds.(b) The mullet has 12 or 13 rows of the lateral-line organs on the flank skin. The scale along each lateral-line has a groove at its central part and a single neuromast is located in it. Through a small hole a few lateral-line nerve fibers reach to the end organ. Each groove is discrete and independent so that there is no lateral-line canal. From such structure of the lateral-line system the end organs in grooves are thought to be free neuromasts.(c) On the flank skin of the carp there are free neuromasts and terminal buds as well as canal neuromasts. The terminal buds, however, are not as numerous as in the catfish.