The effects of serotonin altering parasites, serotonin, the anti-depressant fluoxetine, plus two other highly prescribed pharmaceuticals (carbamazepine and diclofenac) on the behaviour of the marine amphipod, Echinogammarus marinus were investigated. Acanthocephalan parasites are known to alter the swimming behaviour in their amphipod hosts through changes in serotonergic activity resulting in increased predation. Behavioural assays were adapted to record changes in phototaxis and geotaxis behaviour in male E. marinus following 7, 14 and 21 days exposure to serotonin and each pharmaceutical compound at 4 concentrations compared to a control (between 10 ng/L and 10 microg/L). E. marinus infected with acanthocephalans parasites had both significantly higher phototaxis and geotaxis scores than those of uninfected specimens. Phototaxis and geotaxis behaviour increased significantly in a concentration-dependent manner with exposure to serotonin. Fluoxetine significantly altered phototaxis and geotaxis activity in what appeared to be a non-monotonic concentration response curve with the greatest behavioural changes observed at 100 ng/L. The main patterns of these behavioural responses were consistent between two trials and the 3 weeks exposure with specimens spending more time within the light and occurring higher in the water column. No obvious trends could be concluded in the phototaxis and geotaxis scores from individuals exposed to carbamazepine or diclofenac as might be expected from their known mode of action. From this study phototaxis and geotaxis behaviour have been observed to be affected by exposure to serotonin modulators. Parasite studies have shown strong links between changes in behaviour and increased predation risk correlating with changes in serotonergic activity. This study has highlighted the potential for highly prescribed anti-depressant drugs to change the behaviour of an ecologically relevant marine species in ways which could conceivably lead to population level effects.