Fluoxetine (Prozac), a drug of the Selective Serotonin Reuptake Inhibitor (SSRI) class, is known to affect organisms across the animals, plants, and fungi kingdoms, generally by blocking reuptake, and increasing available levels, of serotonin. Fluoxetine has been found in many types of aquatic ecosystems and could potentially be altering the behavior of resident organisms. The present study investigated how an environmentally-relevant concentration (1 μg/L) of fluoxetine alters behavior in Daphnia magna, a planktonic crustacean belonging to the Phyllopoda subclass. Serotonin is known to depress inhibitory responses in a variety of organisms. We tested if fluoxetine exposure in D. magna in the presence, or absence, of soluble predator chemicals (kairomones) affected phototactic behavior. We found that D. magna exhibited a statically-significant (p < 0.05) preference for dark, over light, in control, fluoxetine, or kairomone groups; however, animals exposed to both fluoxetine and kairomones lost significant (p = 0.09) negative horizontal phototaxis. D. magna are a model invertebrate organism, our results shed light on how fluoxetine may be affecting behavior in other types of organisms in aquatic ecosystems.