Abstract 1. 1. Catecholamines were localized in the branchial nerve within the gill epithelium of Mytilus edulis and Elliptio complanata. 2. 2. Denervation or treatment with reserpine depleted catecholamines within the branchial nerve in both species. 3. 3. Application of a monoamine oxidase inhibitor (nialamide) increased catecholamines in the gill. 4. 4. Chemical assays for catecholamines revealed large amounts of dopamine in E. complanata; no dopamine was detectable in M. edulis; norepinephrine was not detectable in either species. 5. 5. Intact gill was found to metabolize [ l- 3H]DOPA and [ l- 3H]tyrosine to [ 3H]dopamine but not to [ 3H]norepinephrine. 6. 6. This and other evidence implicate dopamine as a probable inhibitory agent in the control of ciliary motion of molluscs.