We evaluated behavioral responses of zebrafish Danio rerio exposed to sublethal concentrations of sodium hypochlorite using an image analysis biomonitoring system (IABS). First, the limits of normal variation in swimming activity of zebrafish were determined by monitoring traveled distance of 40 control fishes using the IABS. An acute toxicity test was performed to determine the LC(50(24 h)) for D. rerio to NaOCl. To evaluate the toxic effects in swimming activity, 32 fishes were exposed to 40%, 30%, 20%, 10% of the LC(50 )and 32 were used as control using the IABS. We considered toxic concentrations where more than 10% intervals of the treated group were below the limits of normal variation and were significantly different from the control group. Two main responses were observed: an escape response (increased swimming activity) at 10% treated group, a gradual decrease in swimming activity from the 20% of the LC(50) on, and an avoidance response at higher concentrations. The response of the 20% treated group were considered as a NOAEL and responses of the 30% and 40% treated groups indicated significant hypoactivity (adverse effect). This behavioral biomonitoring system has proven to be a useful tool to detect sublethal toxicity that could be incorporated in biomonitoring protocols in Brazil.