The effects of sublethal concentrations of copper on the structure and activity of schools of the Atlantic silverside Menidia menidia were studied. A computerized video technique was employed to provide three-dimensional analysis. Ten experiments were conducted in which the behavior of a school exposed to from 0 to approximately 100 μg/L of copper was compared to the behavior of the same school on the previous day (control). Fish schools exposed to copper differed significantly (P < 0.05) from control schools in distance to nearest neighbor, swimming speed, mean direction of travel, rate of change of direction, and depth of the school in the tank. At low concentrations of copper, schools increased their swimming speed, decreased their rate of change of direction and their nearest-neighbor distances, and swam more in parallel orientation than schools under control conditions.