This work aims to study the usage of technical vocabulary in expository texts, written by 3rd grade children, from Córdoba, Argentina. The texts were produced in the frame of a didactic sequence for the learning of a textual genre: “encyclopedia entries of animals”. The sequence considered the genres aspects and their characteristics. Initial productions -written at the beginning of the sequence- and final productions were compared in order to evaluate the differences. The results showed important progress in the usage of specific vocabulary, concerning classification, physical description and habitat description of the animals. A reduction in the usage of colloquial and familiar vocabulary was observed. The article also discusses the incidence of interactions with adults and the role of didactic activities in the frame of a genre sequence.