This study used a randomized experimental design to examine the relationship between teacher-student ratio and literacy learning outcomes for experienced intervention teachers working with the most at-risk first-grade students. Eighty-five Reading Recovery teachers, working with 170 students, each taught in a 1:1 and a small-group instructional format with teacher-student ratios of 1:2, 1:3, or 1:5. The at-risk students were assessed at pretest and posttest with the six subtests of "An Observation Survey of Early Literacy Achievement" and the Slosson Oral Reading Test-Revised (SORT-R). The 1:1 instruction yielded significantly higher outcomes than the combined small-group conditions on 8 of the 9 measures. The small group conditions did not differ significantly from one another, but a trend analysis indicated a reduction of literacy performance as group size increased. Implications are discussed for a comprehensive Response to Intervention approach to optimize literacy outcomes and reduce achievement gaps for struggling beginning readers. (Contains 2 figures and 6 tables.)