Dialogical Self Theory, co-regulation, and foundational movement analysis are used to present a description of the development of the dialogical self during the first five months of life using observations of two mother-infant dyads. Susan and her mother illustrate normative emergence of the dialogical self. Susan's I-positions emerge through positive interactions with her mother, for example, through body positioning and dialogue in a flexible yield–push pattern. Peter, another infant we observed, and his mother show how the development of the dialogical self may be disrupted or delayed as rigid boundaries are formed between the mother and infant. Peter's tendency to withdraw, coupled with his mother's persistence at diminishing his positions, results in both avoidant and ambiguous monological positions in relation to his mother. © 2012 Wiley Periodicals, Inc.