The purpose of this study was to teach "conservation of number" (i.e., the idea that a given number of objects remains constant regardless of changes in spatial arrangement) to a group of kindergartners who had not yet acquired the concept. Sixty "nonconserving" students, evenly divided between lower- and middle-class SES, were selected. Half were given individual instruction in conservation of number over one week; the other half were not. All subjects were also given tests for estimates of their verbal comprehension. Results showed significant and durable learning of the concept and language understanding in the experimental group among both lower- and middle-class children. Implications for curriculum, i.e., the desirability and feasibility of teaching these concepts to very young students, are discussed. It is noted that, "without direct instruction, children will usually acquire the concepts of conservation eventually. However, especially for lower-class children, if these concepts are not taught, they may be acquired at such a late stage as to seriously hamper the child's understanding of formal math operations."