This work presents an analysis of the enunciative strategies articulated by a teacher in an 11th grade Chemistry classroom, where the concepts of exo and endothermic processes and enthalpy were discussed. The characterization of these strategies is oriented to the configuration of speech genre of Science classrooms, according to a Bakhtinian view. The used methodology employed the categorization system proposed by Mortimer, Massicame, Buty e Tiberghien (2005 a e b), which was applied to video recorded classroom data, in real time, using a software developed by IPN-Kiel, the Videograph®. The results presented in this paper comprises the time percentages and frequencies of each category of the system, that resulted from this work with the software, and also a microanalysis that, ratified by the percentages, allows to understand how the utterances raises from the enunciative strategies employed by the teacher.