Much research in social psychology has shown that otherwise helpful people often fail to help when bystanders are present. Research in developmental psychology has shown that even very young children help, and that others’ presence can actually increase helping in some cases. In the current study, in contrast, 5-year-old children helped an experimenter at very high levels when they were alone, but significantly less in the presence of bystanders who were potentially available to help. In another condition designed to elucidate the mechanism underlying the effect, children’s helping was not reduced when bystanders were present but confined behind a barrier and thus unable to help (a condition that has not been run in previous studies with adults). Young children thus show the bystander effect, and it is not due to social referencing or shyness to act in front of others, but rather to a sense of a diffusion of responsibility.