We report the effects of aqueous extracts of the microalga Skeletonema costatum Greve (Cleve) (Bacillariophyceae) on spontaneous motor activity, rectal temperature, motor coordination, amphetamine-induced hypermotility, exploratory behaviour, muscle relaxation, catalepsy, conditioned avoidance responses, oxotremorin-induced cholinergic syndrome, and pentylenetetrazole-induced convulsions. The S. costatum extract at dosages of 310 and 620 mg/kg works like an antidopaminergic drug with anticholinergic properties, that does not induce catalepsy and with a notable muscle relaxing activity.