This study evaluated Earobics Step 1, a commercial literacy program, and examined whether impact varied with children's school readiness and classroom instruction. Participants included 247 kindergarteners from 37 classrooms in nine Title 1 schools. Children were randomly assigned to receive 21 weeks of computerized instruction with Earobics or Building Blocks, a commercial mathematics program. Tests of phonological awareness (PA), graphophonemic knowledge, word reading, and vocabulary were administered at three points during the year, and standardized reading tests were administered at the end of year. Earobics did not consistently impact any outcome across children. However, it moderately accelerated PA development among children who started kindergarten with poor emergent literacy, and it positively impacted reading outcomes, but not PA, among children with relatively advanced emergent literacy. Earobics accelerated PA, graphophonemic knowledge, and reading development in classrooms where the teacher provided lots of instruction targeting oral language, PA, phonics, and writing. However it had no impact in classrooms where there was relatively little instruction targeting these skills or in classrooms where there was a lot of small group reading instruction.