Competent larvae of the black scallop Chlamys varia (L.) were treated with the neurotransmitter gamma-amino-n-butyric acid (GABA), the catecholamine epinephrine, and 3-isobutyl-1-methylxanthine (IBMX), at different concentrations and times of exposure, to test the effects of exposure to the compounds on larval settlement. After 24 h, maximum percentage of settlement (>30%) was induced by 10⁻⁵ M epinephrine. After 48 h, maximum settlement rate was also achieved by 10⁻⁵ M epinephrine (>54%). Exposure to 10⁻⁶ M epinephrine and to 10⁻⁶ M GABA also induced significantly higher larval settlement than in control larvae. By contrast, IBMX failed to induce significantly higher settlement rates than in the control larvae. Epinephrine and GABA were identified as active inducers of settlement in C. varia, and were not toxic to black scallop larvae. This is the first report in which the involvement of epinephrine and GABA in the settlement of C. varia larvae is described.