We present findings from a randomized controlled trial of "Descubriendo la Lectura" (DLL), an intervention designed to improve the literacy skills of Spanish-speaking first graders, who are struggling with reading. DLL offers one-on-one native language literacy instruction for 12 to 20 weeks to each school's lowest performing first-graders. Examining literacy outcomes for 187 students, hierarchical linear model analyses revealed statistically significant effects of student-level assignment to DLL on all 9 outcomes evaluated. Impacts were as large as 1.24 standard deviations, or a learning advantage relative to controls exceeding a full school year of achievement growth. The mean effect size of d = 0.66 across the nine literacy measures is equal to approximately two thirds of the overall literacy growth that occurs across the first-grade year.