Direct tests were performed on the concs. and exposures of a variety of metallic salts necessary to kill barnacles (Balanus balanoides and B. eburneus). The toxicities of mercury, cupric citrate, cupric tartrate, cupric salicylate, and cupric aminobenzoate were found to be slightly less than the toxicity of basic cupric carbonate. The toxicity of silver is about equal to that of basic cupric carbonate, but the toxicity of zinc is very much less. The rate of killing of barnacles by cupric citrate is proportional to the cone, of the toxic over the range tested. An extremely high cone, of Cu or of Hg salts was necessary to prevent the metamorphosis of cyprids attached to glass plates. The results show the difficulty of preventing the initial attachment of cyprids, or their metamorphosis, by the use of copper paints. Moderate cones, of cupric citrate seriously retard the development of the newly metamorphosed barnacles and prevent the 2d step in attachment, namely, the formation of the cemented calcareous base. Exposure of the newly metamorphosed barnacles to very low cones, of cupric citrate accelerated development beyond that of the normal animals. The soft tissues of adult barnacles normally contain a much higher cone, of Cu than does sea water. When placed in solns. of cupric citrate, barnacles absorbed >100 times the normal Cu content of the tissues. In no case were barnacles killed by the absorption of 10 times the normal content. In some cases barnacles which had absorbed 0.5 mg. to 1.09 mg./-g. from toxic solns. revived when replaced in fresh sea water. When replaced in fresh sea water, barnacles can eliminate from their tissues as much as 2/3 of the Cu which has been absorbed from toxic solns. Mussels (Mytilus edulis) are more sensitive to poisoning by cupric citrate than barnacles. When exposed to Cu solns., mussels take up Cu more rapidly than do barnacles, and when replaced in fresh sea water, they eliminate it from their tissues more rapidly and extensively. In many cases in which a considerable portion of the Cu was eliminated, the mussels nevertheless succumbed subsequently.