Due to its abstract, theoretical nature, the mole concept has been recognized as one of the most difficult topics to teach and learn within the chemistry curriculum. The purpose of this study was to chronicle the development of high school students' conceptions of the mole following a period of instruction in a chemistry class. This investigation is part of an ethnographic study of curriculum modulation that documents class activities through participant observation. An analysis of the chemistry textbook's presentation of the mole concept and a comparison to the teachers' instructional techniques are discussed. Students' understandings of the mole concept are examined in relation to the textbook presentation and teacher influences. The findings indicate that students may fail to construct meaningful understandings of the mole concept for the following reasons: (1) inconsistency between the instructional approaches of the textbook and teacher; 2) confusing mole concept vocabulary; (3) students' math anxiety and proportional reasoning ability; (4) learners' cognitive levels; and (5) lack of practice in problem solving. (Author/DDR)