Researchers report that early intervention can reduce and eliminate reading difficulties for early readers. What Works Clearinghouse evaluated several phonological awareness interventions, but only one had positive effects on alphabetics and one had potentially positive effects on phonological processing and alphabetics. Additionally, the most recent National Assessment of Educational Progress reported that only 36 percent of fourth grade students performed at or above the proficiency level of reading in 2015. Researchers must continue rigorous evaluations to identify effective interventions for students in primary grades. Researchers and educators have implemented Phonological Awareness and Vocabulary Intensive Intervention (PAVII), an early literacy intervention, but no study has tested its efficacy on a native English speaking population using a rigorous causal research design, and no study assessed treatment acceptability. The purpose of this study was to evaluate this intervention's effects on first grade students' phonemic awareness and phonics skills and to evaluate treatment integrity and treatment acceptability. The participants were selected from a suburban elementary school in Southern California. The efficacy of the intervention was tested using a regression discontinuity design. The treatment integrity and acceptability outcomes were high, although the treatment acceptability results were not as high as anticipated. The RDD results indicated that PAVII did not have a casual main effect on students' phonemic awareness or phonics skills. However, the effect of the intervention on phonemic awareness skills was moderated by students' initial phonemic awareness: The intervention had significantly greater effects on students with initially higher phonemic awareness skills. This suggests that the effect of PAVII on phonemic awareness skills depends on students' initial phonemic awareness skill level. Future research is needed to examine whether PAVII combined with direct phonics instruction improves the intervention's efficacy. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]