This study was conducted to determine whether the Chapter 1 instruction provided to low-achieving students in the Oklahoma City Public Schools had a detectable effect on student academic growth. Reading and math achievement test scores of Chapter 1 students in grades 2 to 8 were compared with those of a matched group of low-achieving non-Chapter 1 students over 2 years (1981-82 and 1982-83). Chapter 1 students were provided with 30-50 minutes per day of extra remedial instruction in reading or math, and the achievement of both groups was measured by the California Achievement Test (CAT). An Analysis of Covariance (ANCOVA) procedure was used to address aptitude-treatment interaction (ATI) questions of whether Chapter 1 treatment affected students differently, depending on their pretreatment achievement standing. Results, portrayed by a series of graphs and tables, indicate that for nearly all grades, 23 out of 28 within-grade comparisons showed slope differences in the same direction. These results indicate that initially lower achievers benefit more from Chapter 1 treatment than initially higher achievers. Implications of these findings for Chapter 1 selection criteria are discussed. (TE)