Naturally occurring interactions recorded during participant observation in two preschools were analyzed in order to develop a distinctively sociological theory of gender identity acquisition. Attention was focused on the use of sex-categorical terms and the "grammar" of sex categorization practices underlying this usage. The analysis revealed that young children initially use the terms "girl,""boy," and "baby" as a collection of categories similar in form to the collection "mommy,""daddy," and "baby." Because adults typically associate children's sex categorical identity with competence, children appropriate this identity in order to avoid the discrediting identity of "baby" and become, by implication, committed to its behavioral confirmation. In attempting to behaviorally confirm this, children gain an awareness that the sex categories are mutually exclusive while sex-specific appearance management, pronoun usage, and the verbal instructions of others convey to them that sex categorization is lifelong. By the end of the preschool age period, children actively seek guidance concerning the behavioral expression of their sex-categorical identity and actively attempt to confirm it in interaction with others. Gender identity acquisition is not only a process of individual development; it is also the process whereby the objectivity of gender within this society is intergenerationally transmitted. (Author/RH)