Preschool-age children identified as at risk for later reading difficulties can benefit from supplemental, small-group emergent literacy intervention. As such interventions become commercially available and marketed to preschool programs, it is important to understand their impacts when implemented by intended end users under routine conditions. In this study, we examined the effects of the Nemours BrightStart! (NBS!) intervention on children's emergent literacy skills when implemented by teachers and community aides in authentic preschool classrooms. We randomly assigned 98 classrooms to one of three conditions (NBS! teacher-implemented, NBS! community aide-implemented, or control). Children enrolled in these classrooms who met eligibility criteria and were identified as at risk via an early literacy screener (n = 281) completed pretest and posttest emergent literacy assessments; those assigned to NBS! conditions received intervention from their classroom teacher or a community aide affiliated with a local kindergarten-readiness initiative. Intent-to-treat analyses showed no significant impacts of NBS! on any outcome, and an instrumental variable, as-treated approach showed one significant intervention effect on letter writing. Consequently, we did not replicate results of prior highly controlled efficacy trials. Findings have implications for revising the NBS! theory of change, conducting dosage and as-treated analyses, and moving research-based interventions toward scale-up.