Teachers play a critical role in deciding what curricula are used in their classrooms. We examine the factors that teachers describe as influencing their sustainment or discontinuation of a literacy curriculum, "Zoology One," following their participation in a randomised controlled trial (RCT) examining the curriculum's efficacy. This study was conducted in a large urban district in the United States and the curriculum was implemented in kindergarten classrooms with children who are typically 5 and 6 years old. One year after their participation in the RCT, teachers who had been randomised to implement "Zoology One" during the efficacy study (N = 19) participated in interviews about their ongoing use of the curriculum. We analysed the interview data using a staged coding approach to understand the factors that teachers described influencing their sustainment or discontinuation of the curriculum. We used a multilevel framework to organise results across three levels: individual-level, school-level and macro-level. Results indicate that teacher perceptions of the curriculum, including those related to its effectiveness and age-appropriateness, contributed to sustained use. At the same time, some teachers' perceptions that the curriculum approaches were not sufficient for their students led them to discontinue some curriculum components. The implementation climate at their school, as well as the widespread adoption of a particular phonics programme within the school district, also influenced teachers' sustainment or discontinuation. Taken together, these results highlight the importance of teachers' perceptions of a curriculum as well as the critical role that the school and district context play in curricular sustainment.