Human antidepressants have been shown to have a wide variety of effects on aquatic mollusks. The antidepressants venlafaxine (“Effexor”) and citalopram (“Celexa”) are human pharmaceuticals detected in effluent from wastewater treatment plants. We report on the acute effects of venlafaxine and citalopram on the attachment ability of two freshwater snails, Leptoxis carinata and Stagnicola (=Lymnaea) elodes. In laboratory experiments, venlafaxine induced significant foot detachment from the substrate in L. carinata at nominal concentrations as low as 313 pg L⁻¹ (10⁻¹² M) and in S. elodes at 31.3 ng L⁻¹. Citalopram was equally potent in L. carinata, inducing significant foot detachment in 405 pg L⁻¹ (10⁻¹² M), but slightly less potent in S. elodes inducing significant foot detachment in 4.05 µg L⁻¹. In all cases of foot detachment from the substrate, snails were not able to reattach while exposed to the drugs. The lowest observed effect concentrations (LOEC) for venlafaxine (313 pg L⁻¹) and citalopram (405 pg L⁻¹) are about 700× and 25,000×, respectively, lower than the concentrations of these two antidepressants recently reported from North American and European wastewater treatment plants. These are the lowest concentrations of any antidepressant reported to have a detrimental effect on any aquatic animal. The possible physiological mechanism of this toxicity and the potential ecological impacts of these and other antidepressants on aquatic snails and their communities are discussed.