This article examines the relationship between the mathematical knowledge of 438 K-8 California teachers and the demographics of the schools in which they work. To measure mathematical knowledge, we used a series of multiple-choice problems meant to represent both the content teachers teach and the specialized knowledge of mathematics that teachers might possess. Teachers in schools with higher proportions of low-SES and Hispanic students performed more poorly on these measures than did teachers from other schools. Implications for policy and for further research are discussed. (Contains 5 tables, 4 figures and 2 notes.) [The research reported in this article was supported in part by the National Science Foundation's Interagency Educational Research Initiative to the University of Michigan, the National Science Foundation's Research on Learning and Education, and by the University of California Office of the President.]