Guided by a theoretical framework emphasizing the importance of fidelity of implementation (FOI), this paper explores how teachers of grades 3, 4 and 5 implemented an early algebra intervention, and the relationship between FOI and student learning. The data shared in this paper come from a longitudinal experimental research project in which 3,208 students from 46 schools were followed for three years. Videotaped classroom observations, our primary source of FOI data, were coded to capture teachers' instructional practices, and an algebra assessment was given to assess student performance in response to the teachers' implementation of our instructional intervention. Results revealed a significant positive relationship between aspects of teachers' implementation and their students' performance. [This paper was published in "Journal for the Study of Education and Development" v42 n3 p523-569 2019.]