Texas has been an important player in the emergence of the charter school industry. We test for a competitive effect of charters by looking for changes in student achievement in traditional public schools following charter market penetration. We use an eight-year panel of data on individual student test scores for public schools students in Texas in order to evaluate the achievement impact of charter schools. We estimate a model that includes student/campus spell fixed effects to control for campus demographic and peer group characteristics, and to control directly for student and student family background characteristics. We find a positive and significant effect of charter school penetration on traditional public school student outcomes.