Starting in the 2013-2014 school year, I conducted a randomized field experiment in forty-six traditional public elementary schools in Houston, Texas designed to test the potential productivity benefits of teacher specialization in schools. Treatment schools altered their schedules to have teachers specialize in a subset of subjects in which they have demonstrated relative strength (based on value-add measures and principal observations). The average impact of teacher specialization on student achievement is -0.11 standard deviations per year on a combined index of math and reading test scores. Students enrolled in special education and those with less experienced teachers demonstrated marked negative results. I argue that the results are consistent with a model in which the benefits of specialization driven by sorting teachers into a subset of subjects based on comparative advantage is outweighed by inefficient pedagogy due to having fewer interactions with each student. Consistent with this, specialized teachers report providing less attention to individual students (relative to non-specialized teachers), though other mechanisms are possible.