In this study, we analyzed mothers' speech and play behavior with their 6-, 9-, and 14-monthold sons and daughters. Thirty-six infant-mother dyads participated in a 10-min free-play session with gender-neutral toys. No sex differences were found in the infants' behavior, but sex differences were found in mothers' verbal behavior and level of engagement. Mothers of daughters made more interpretations and engaged in more conversation with their daughters, whereas mothers of sons made more comments and attentionals, which were typified by instructions rather than conversation. Furthermore, mothers interacted more with their daughters than with their sons across all ages. Overall, these results demonstrate that mothers transmit different messages to their male and female infants, both through language and interaction, which may contribute to infants' gender role development. KEY WORDS: gender socialization; infant development; infant gender; play; parent-infant interaction.Most developmental theorists acknowledge socialization as at least a contributing factor to the development of gender differences in children. For example, social learning theory posits that children learn behavior through reinforcement and modeling (Bandura & Walters, 1963;Mischel, 1966). With respect to gendered behavior, children might be praised for behaving in "gender-appropriate" ways (e.g., girls playing with dolls, and boys playing with trucks), and scolded for behaving in "genderinappropriate" ways (e.g., girls playing with trucks, and boys playing with dolls). Similarly, gender role socialization theory posits that different people and objects in a child's environment provide rewards and models that then cause the children to shape their behavior to fit with gender norms (e.g., Block, 1973). Gender-schema theory uses these same principles of social learning to explain how people acquire masculine and feminine gender categories, and what characteristics and behaviors people associate with those categories (Bem, 1981).