Serotonin (5-HT) was detected in various body tissues of the scallop using both immunocy-tochemistry and high performance liquid chromatography with electrochemical detection (HPLC-ED). Much of the 5-HT in the body appears to originate from nerve cells in pedal and cerebral ganglia. Neuropilar regions of these ganglia are also abundant in 5-HT. Far fewer 5-HT containing cells reside within the parietovisceral ganglion, although the accessory lobe contains numerous such cells. The lateral lobes of the parietovisceral ganglion are richly innervated with 5-HT containing fibres. Immunoreactive fibres were also found in the heart, kidneys, labial palps, gills and various muscles. The digestive tract contained numerous immunoreactive cell bodies. Immunoreactive fibres were found principally around the collecting tubules of both the testes and the ovaries in the spring, but also around the germinal acini by autumn. HPLC-ED was used to quantify the amount of 5-HT present in the above-mentioned tissues in the spring. Contents ranged from about 200 pg/mg wet weight of labial palps to about 2500 pg/mg wet weight of foot. It is concluded that 5-HT has the potential to influence numerous physiological processes in the scallop including reproduction, respiration, feeding, digestion, excretion, movement and visual information processing. Both seasonal variations and artificially induced changes in systemic 5-HT levels are therefore likely to have wide ranging effects.