In response to the establishment of standards by states and professional organizations, many local school districts have adopted a standards-based curriculum. The expressed purpose of standards is to improve student academic performance by providing teachers with a common sequence of targets at which to aim instruction. In this study, we examine unintended consequences of a school district's standards-based reform effort. Though the district intended to enhance student achievement and equalize educational opportunities for students, it instead caused the evolution of what can be called standards gaps, which resulted in differentiated curriculum and instruction along lines of students' academic ability.