We experimentally compare two modes of in-service professional development for South African public primary school teachers. In both modes teachers received the same learning materials and daily lesson plans, aligned to the official literacy curriculum. Students exposed to two years of the program improved their reading proficiency by 0.12 standard deviations if their teachers received centralized Training, compared to 0.24 if their teachers received in-class Coaching. Classroom observations reveal that teachers were more likely to split students into smaller reading groups, which enabled individualized attention and more opportunities to practice reading. Results vary by class size and baseline student reading proficiency.