While school choice may enhance competition, incentives for public schools to raise productivity may be muted if public education is imperfectly substitutable with alternatives. This paper estimates the aggregate effect of charter school expansion on education quality while accounting for the horizontal differentiation of charter programs. Our research design leverages variation following the removal of North Carolina’s statewide cap to compare test score changes for students who lived near entering charters to those farther away. We find learning gains that are driven by public schools responding to increased competition from non-horizontally differentiated charter schools, even before those charters actually open.