Behavior can depend profoundly on the social context, as individuals behave differently in a dyad than when in a larger group. Here, we asked whether trace levels of cadmium, a common industrial pollutant, influences the exploratory behavior of zebrafish (Danio rerio) differently when the fish were tested in pairs or in the context of a larger group of untreated fish. We found no difference in the exploratory behavior of cadmium-treated fish and water-treated controls, when those fish were tested in pairs. In the context of a larger group of 6 fish, however, we observed a striking difference. Groups that contained a pair of cadmium-treated fish approached and remained near a novel stimulus more than did groups containing a pair of water-treated control fish. The difference in group, but not pair, behavior suggests that environmental pollutants can have indirect effects on group behavior.