This study was designed to investigate the effects of embryonic exposure to methylmercury (meHg) on social behavior of larval mummichogsFundulus heteroclitus. Embryos from two sites (polluted and unpolluted) were exposed to 5 and 10 μg meHg/L during development; after hatching, the larvae were transferred to clean seawater. Testing was performed weekly for 4 weeks after hatching. The variables examined were distance between individuals, duration of parallel swimming, and number of collisions or repels. Data indicated that mercury exposure significantly increased the number of collisions by larvae from the unpolluted reference population but not from the polluted population. This effect disappeared by 4 weeks after hatching, indicating that the damage to the developing nervous system was reversible. No trends toward increased parallel swimming or decreased distances between individuals (behaviors that would reflect the beginning of schooling) appeared during the 4 weeks. This indicates that schooling behavior in this species develops when larvae are older than the ones tested.