This work strives to diagnose and comprehend what happens inside the classroom. The data collecting methodology involved: interviews, directly observation and copies of didactic material used by the students in mathematics lessons of primary schools, at Rio de Janeiro, from the year 2002 to 2004. We analyzed 424 lessons of 116 teachers, making possible the description of main observed lessons´ characteristics using content analyses. As a result, I discuss the practices of teachers in action and identify recurring characteristics, analyzing: the materials and resources selected, the activities used, the relations inside the classroom, the complex problems of teachers, and, finally, the conceptions of mathematics´ teaching that appear to support the practices of the group of teachers studied. The analysis also shows the power of the professional knowledge, built over the practice, and the influence of the experience as a student in the future teacher s formation, showing clearly the existence of a teacher´s culture that preserves acquired conceptions and fits recommendations for change into old practices.