To examine how classroom conditions such as students' mean aptitude, materials used, and instructional time influence the amount of reading material covered, 12 first grade classrooms were each observed for 12 days over a 36-week period. Students' reading readiness and word recognition skills were tested prior to observation while group mean aptitude, reading group size, and number of phonics concepts and new words introduced in the basal reading series were recorded during the study. Time spent on basal instruction, teacher-supervised reading group activities, and phonics work was determined by direct observation. Data corroborate earlier findings that the amount of material covered during a reading class--a significant predictor of reading achievement--was influenced by two main sets of conditions: characteristics of the group, particularly its mean aptitude, and conditions of the classroom, especially difficulty of materials and associated time conditions. (MM)