Science classrooms in the United States have become more diverse with respect to the variety of languages spoken by students. This qualitative study used ethnographic methods to investigate the discourse and practices of two ninth grade science classrooms. Approximately 44% of students included in the study were designated as English learners. The present work focused on addressing the following questions: 1) In what ways is science discourse taken up and used by students and their teacher? 2) Are there differences in how science discourse is used by students depending on their English language proficiency? Data collection consisted of interviewing the science teacher and the students, filming whole class and small group discussions during two lesson sequences, and collecting lesson plans, curricular materials, and student work. These data were analyzed qualitatively. Findings indicated that the teacher characterized science discourse along three dimensions: 1) the use of evidence-based explanations; 2) the practice of sharing one's science understandings publically; and 3) the importance of using precise language, including both specialized (i.e., science specific) and non-specialized academic words. Analysis of student participation during in-class activities highlighted how students progressed in each of these science discourse skills. However, this analysis also revealed that English learners were less likely to participate in whole class discussions: Though these students participated in small group discussions, they rarely volunteered to share individual or collective ideas with the class. Overall, students were more adept at utilizing science discourse during class discussions than in written assignments. Analysis of students' written work highlighted difficulties that were not visible during classroom interactions. One potential explanation is the increased amount of scaffolding the teacher provided during class discussions as compared to written assignments. In the implications section, I provide science teachers with recommendations regarding how to promote science discourse in their classrooms. Specifically, teachers should provide students structured opportunities to practice science discourse, require students to use both written and oral modalities in assignments, and offer timely feedback to students regarding their progress in developing their science discourse skills. How this study contributes to the research base on the teaching of science and English learners will also be described. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]