An economic framework, which combines the theories of human capital and internal labor markets within a probabilistic matrix, is developed to analyze teacher mobility. Among the implications of this framework that received confirmation when tested with data for the San Diego school system are the following: (1) Since teaching assignments within school systems do not differ in terms of salary, internal mobility of teachers is governed by nonpecuniary differences, such as those associated with student socio-economic status (SES). As a result (a) new teachers tend to be placed in low SES schools; (b) teachers tend to move away from low SES schools toward higher SES schools; (c) teachers with the most experience are the least likely to change assignments. (2) As a consequence of these mobility patterns, higher SES schools have faculties with relatively greater experience and educational attainment, resulting in positive associations between these teacher characteristics and student achievement. 38 pp. Ref.