1. Dactyl chemoreceptors were electrophysiologically demonstrated in Carcinides maenas, Libinia emarginata, Callinectes sapidus, and Paguris pollicaris. 2. In Carcinides the receptors respond to a boiled dialysate of Mytilus, and to 1-glumatic acid at dilutions in excess of 5.0 x 10-5M. Derivatives of glutamic acid and other amino acids are less stimulatory. 3. Optical isomers of glutamic acid, aspartic acid and leucine are distinguished by small populations of Carcinides dactyl receptors.