Children's ability to distinguish among those regularities in the environment governed by social convention and those governed by physical law was investigated in this study. 75 children from the first, third, and fifth grades were each asked a series of questions pertaining to 6 topic areas. 4 of these topics dealt with conventionality, one with moral rules, and another with physical laws. In each topic area such issues as the child's understanding of the convention, rule, or law and the child's belief in the universality of the convention, rule, or law were addressed by similar questions. 4 conclusions were drawn from the results: (a) Children's ability to distinguish between social convention and physical law increases with age. (b) In learning to make this distinction, many children pass through an intermediate stage where they believe, incorrectly, that both physical laws and social conventions can be changed. (c) Children s understanding of conventionality develops as an organized whole. (d) Some conventions are more difficult to change than others. Children are more reluctant to change moral rules than conventions.