This study reports the application to classroom dialogue of the Thematic and Structural Analysis (TSA) Technique which has been used previously in the analysis of text materials. The TSA Technique identifies themes (word clusters) and their structural relationship throughout sequentially organized material. Dialogues from four Year 8 science classrooms are analyzed using the TSA Technique and the resulting structures rank-ordered on the basis of the number, quality and organization of themes. The structure of the dialogue was the most influential predictor of achievement. Implications of the findings for teaching and teacher education are discussed. An appendix includes a list of the five measuring instruments used in the study. There are 28 references cited. (Author/CW)