The ecological concepts of activity structure and activity segments were central to this examination of observational data attained in a three-week period from 18 fifth-grade math classes and 17 fifth-grade social studies classes in school districts in the greater Chicago region. Classroom observations were conducted by pairs of observers: one observer recorded information about the activity structure and behavior setting; the second observer used a strict time-sampling rotation method to study a subset of eight children in each classroom. In the analysis only instructional (as opposed to transitional) segments were examined. The data base consisted of 461 math segments and 474 social studies segments. The frequency distributions of various ecological features of segments were examined; a chi-square test compared frequencies across subject matters. A similar analysis involving mean durations of segments with different properties and mean occupancy time (length X the number of students in the segment) was performed using ANOVA procedures. Results indicated that math and social studies classes differed with regard to distributions of instructional formats. Overall, more variety in format was found in social studies than in math. Additional results were discussed. (Author/RH)