Urban development is a form of anthropogenic environmental disturbance that globally threatens waterways by altering the hydrology and water quality. We are assessing how increased urbanization near rivers of Puerto Rico impacts the aquatic biota. Here we compare the abundance of Macrobrachium carcinus prawns in representative rivers and asses the effects of water chemicals found in those near urban areas on prawn locomotor activity and aggression.A monthly census of prawns along different gradients of urban intensity was conducted in four rivers of the island. The relative number of prawns collected by traps was significantly higher (144) in the non‐urban river, compared with 0, 0, and 2 prawns in comparable areas of three urban rivers.Prawns from non‐urban rivers were exposed in the lab to concentrations the EPA defines as limits for drinking water of specific phthalates (0.006 mg/L) and metal contaminants (Cr3+;0.1 mg/L, Cd2+;0.005 mg/L, Mn2+;0.05mg/L). All contaminants increased locomotion and DBP, Cr3+ and Cd2+ increased total area covered by prawns in the tank, and altered their locomotor patterns. Prawns injected with DBP ventured more towards the center than remaining near edges. Submissive prawns injected with DBP, Cr3+, Cd2+ and Mn2+ also escalated aggression levels.Results suggest that contaminants in urbanized watersheds are associated with prawn assemblage degradation and also have an impact on locomotion and aggressive behavior. These findings serve as important indicators of how low levels of anthropogenic contaminants, such as phthalates and metals, can impact behaviors essential for survival.