The purpose of this study was to determine the nature of teachers' implicit in-action mental models about children's minds and learning, as inferred through the ways they teach. The work was based on the theoretical works of D. Schon, L. Shulman, and P. N. Johnson-Laird. Study participants included 24 student, novice, and experienced teachers. All teachers taught the same first-grade arithmetic lesson. Teacher behaviors and the inferred teachers' in-action mental models are described. Findings indicate that teachers have an in-action mental model of children's minds and of how learning takes place in children's minds and that this mental model directs their teaching. This model, which is implicit and intuitive, reflects an understanding of the mind shared by teachers from all three groups. Four implications for teacher education are: (1) the in-action mental model has profound influence on how teachers teach; (2) preservice and inservice teachers understand what is being taught in courses and workshops that relate to children's learning, via their in-action mental model; (3) teachers are not aware they hold this model, and in order for the implicit in-action model to be discussed, it must become explicit; and (4) the tool developed to describe teachers' in-action mental models of children's minds and learning can be used in teacher education both as a diagnostic tool and as a tool for instruction. (Contains seven references.) (ND)