Abstract 1. Bathing freshwater mussels in water containing ergotamine or methylergometrine induced rhythmic coordinated movements of the foot and shell maintained in a stereotyped form for hours. The effect was weak at 10 −9mol/1, moderate to strong at 1–4 × 10 −8 mol/1, and irreversible to lethal at 5–8 × 10 −8 mol/1. 2. Bathing mussels in serotonin (10–20 μmol/1) induced similar movements. They closely correspond to the well-known digging cycles exhibited by pelecypod molluscs during burrowing locomotion. 3. The serotonin content of central ganglia was not changed significantly in mussels exhibiting the ergot-induced locomotory movements. Ergotamine, up to 10 −5−10 −6 mol/1, did not affect the spontaneous and evoked liberation of serotonin, as well as the serotonin uptake. There is thus little indication of any direct influence of ergots on serotonergic mechanisms.