Although classroom "openness" has been much discussed in recent years, there has been little effort to investigate to what degree this openness occurs within a general sample of classrooms. The purpose of this study is to identify significant attributes of classroom activity and organization relevant to the concepts of "traditional" and "open" and to see whether these attributes can be used to derive meaningful classroom types. Fifty fourth-grade public school classrooms in Montgomery County, Maryland were observed eight times for specific behavioral categories referring to general organization and activity, teacher behavior, student behavior, and classroom atmosphere. Results indicate that there are three basic classroom types of approximately equal numbers. The first type was characterized as "open" and included elements of student autonomy, individualized student-teacher interaction, emphasis on student creativity and involvement, and a warm, friendly atmosphere. The second, more traditional classroom type was characterized by tight teacher control, little student autonomy, cold and critical atmosphere, and an emphasis on individual work on convergent tasks. The third classroom type was a combination of the open and traditional where teachers stimulated student interest by their own relatively flamboyant classroom performance. (Author/DE)