<p>Research has shown how language plays an important role in science education and recommends that teachers give space to students to use scientific discourse to build explanations. This study examines the conceptual learning of primary school students when participating in talk-intensive science lessons. To produce such a dialogic case in the context of a monological-directive tradition of teaching, a classroom intervention was implemented in a Brazilian primary school. Data consisted of pre-post knowledge tests, audio recording of small-group work, video recording of whole-class teaching, and student interviews. Classroom talk was described through quantitative methods which employed discursive measures and a coding scheme. Although statistically significant, only small effects on students’ learning were apparent, even though the lessons were described as highly interactive and moderately dialogic. These findings are discussed in relation to the role of different teaching resources and modalities, talk formats, and classroom culture in promoting the co-construction of knowledge.</p>