lridophores in the dermis of the floating goby, Chaenogobius sp. 2, are motile. Their movements involved centrifugal or centripetal migration of the light-reflecting platelets within the cells. High K+ solution induced dispersion of the platelets in the cells. This response was lost in iridophores in denervated scale preparations. These results suggested that K+ acted on the nerve controlling the iridophores to release the transmitter. Norepinephrine, whirch was assumed to be the transmitter in the nerve, caused the platelet dispersion. The action was inhibited by an alpha adrenergic antagonist, phenoxybenzamine, but not by a beta adrenergic one , propranolol. This indicated that norepinephrine acted on the alpha adrenoceptors on the cell membrane to induce the platelet dispersion. Forskolin, a unique activator of adenylate cyclase , was effective in inducing aggregation of the platelets. Melatonin enhanced aggregation of the platelets. Alpha MSH had no observable effect on the iridophores. Based on these results, it was concluded that Chaenogobius iridophores are under the dual control of the sympathetic adrenergic nerve and melatonin, the hormone from the pineal gland and that anincrease in the intracellular cAMP induces the platelet aggregation within the cells.