The paper analyzes the conversational moves that teachers devote to the treatment of unknown words in elementary school science lessons. The corpus consists of 48 teaching situations recorded in 1st, 3rd, and 5th classrooms of 4 elementary schools located in the province of Córdoba, Argentina, which were selected according to criteria of typical representation. A system of dimensions and categories was designed, inductively, following the constant comparative method. This system made it possible to qualitatively analyze the conversational moves that teachers employ to support the learning of unfamiliar words. Through the Kruskal-Wallis H and the Mann-Whitney U tests, quantitative differences and similarities were identified depending on educational level. The results showed that 1st grade teachers have a greater tendency to correct the unknown word and provide gestural information than 3rd grade teachers. The analysis of the opportunities that children have to learn words in the classroom made it possible to evaluate the quality of teaching situations.Este trabajo analiza los movimientos conversacionales que las maestras dedican al tratamiento de palabras desconocidas durante clases de ciencias de la escuela primaria. El corpus está conformado 48 situaciones de enseñanza registradas en 1º, 3º y 5º grado de 4 escuelas _2 urbanas y 2 rurales_ de la provincia de Córdoba, Argentina. Los datos se analizaron mediante el empleo combinado de procedimientos cualitativos y cuantitativos. Los resultados mostraron que las maestras tienden a corregir y ajustar explícitamente la palabra desconocida y a proporcionar información prosódica durante su enseñanza, sobre todo en 1° y 5° grado. El análisis de las oportunidades que tienen los niños de aprender palabras en el aula permitió evaluar la calidad de las situaciones de enseñanza.