The purpose of this quantitative study was to examine the effects, if any, of a supplemental phonics curriculum, Saxon Phonics, on the reading achievement of first-grade students in one mountain-west, semirural, school district. The design was casual-comparative and ex post facto, and answered the questions: (1) Do students taught using a traditional basal program and students taught using both the traditional basal program and a supplemental phonics program (control vs. treatment) differ on selected end-of-year reading achievement scores (i.e., portions of the DIBELS Next assessment--nonsense word fluency [NWF], oral reading fluency [ORF], and accuracy [ACC])? (2) do any possible interactions among selected variables (i.e., instructional program, gender, and beginning-of-year reading level) exist related to performance differences on end-of-year reading achievement scores among students receiving reading instruction with or without a supplemental phonics program? The independent variables were the instructional program Saxon Phonics, a traditional/basal reading curriculum and the reading levels of low, medium, and high. The dependent variables were oral reading fluency, accuracy and nonsense word fluency, measured by the DIBELS Next assessment. The 2014-2015 and 2015-2016 DIBELS Next data were collected from the school district database upon approval from the Institutional Review Board in January of 2017. A mixed effects model was utilized to explore the relationship between use of the selected supplemental phonics curriculum and selected reading achievement scores of first-grade students. Results revealed that there was no significant difference between the control and treatment groups, though there was a statistically significant improvement of low readers in the treatment group over the control group. [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.]