This article focuses on the capacity of students to develop and assess arguments during a high school genetics instructional sequence. The research focused on the locating distinction in argumentation discourse between “doing science” vs. “doing school” or “doing the lesson” (Bloome, Puro, & Theodorou, 1989). Participants in this classroom case study were high school (9th grade) students in Galicia (Spain). Students were observed, videotaped, and audiotaped while working in groups over six class sessions. Toulmin's argument pattern was used as a tool for the analysis of students' conversation and other frames were used for analyzing other dimensions of students' dialogue; (e.g., epistemic operations, use of analogies, appeal to consistency, and causal relations). Instances of “doing science” and instances of “doing the lesson” are identified and discussed as moments when the classroom discourse is dominated either by talking science or displaying the roles of students. The different arguments constructed and co-constructed by students, the elements of the arguments, and the sequence are also discussed, showing a dominance of claims and a lesser frequence of justifications or warrants. Implications for developing effective contexts to promote argumentation and science dialogue in the classroom are discussed. © 2000 John Wiley & Sons, Inc. Sci Ed84:757–792, 2000.