Avoidant behavior is a characteristic feature post-traumatic stress disorder (PTSD) and is modeled in mammals with predator odor. Light avoidance is a hallmark behavioral reaction in planarians. We hypothesized that planarians exposed to frog extract would display enhanced light avoidance that is prevented by fluoxetine. Enhanced light avoidance (i.e., less time spent in light compartment of a dish split into light and dark sides) after a 30-min frog extract exposure (0.0001-0.01%) manifested 15 min post-exposure, persisted for at least 24 h, and was counteracted by fluoxetine (10 μM). These results suggest conservation of an anxiety-like behavioral phenotype.