Fish schools are believed to provide antipredator benefits to their members. Two potential antipredator benefits, the dilution and confusion effects, of schooling were investigated in the laboratory using banded killifish (Fundulus diaphanus). Individual risk of being attacked and killed by a fish predator (white perch, Morone americana) declined with increasing killifish school size in a manner closely predicted by the dilution hypothesis. Perch were apparently not confused by schooling killifish since fish in groups of one did not suffer a disproportionately greater predation rate than fish in all larger school sizes. However, killifish straggling from the school were preferentially attacked and more successfully captured compared with school members. Schooling in the banded killifish therefore confers a considerable antipredator benefit to individual group members, at least through a dilution effect, and straying from a school has an associated increased risk of mortality to predation, which selects for schooling behaviour.