The aim of the present study was to examine the moral and conventional rules in 40 same-sex and same-age 4-and 5-year-old preschoolers' dyads during their play interaction. Significant gender differences were found both in the dominant social domain and in the prevailing types of rules within each domain. Boys referred to moral rules more often than girls did. In the category of moral rules, boys were significantly more likely to bring up justice and rights issues than girls were. In the category of conventional rules, girls focused on miscellaneous and general conventions and boys on the destruction of property. Moral rules were applied more frequently in conflict situations. Regardless of gender, the rules of justice were the most likely moral rules to engender conflicts.