The teachers union in Mexico, or Sindicato Nacional de Trabajadores de la Educación (SNTE) represents over 1 million members and is the largest in Latin America. This study uses data from the national student tests administered by the Instituto Nacional para la Evaluación de la Educación (INEE), along with data from the Mexican Ministry of Education and other sources, to investigate the relationship between dfferent measures of union influence and primary student test scores in Mexico. It found that union membership is not merely a function of state student enrollments, but of political and other factors. For example, the number of teaching positions (-plazas') appears to increase with each election period, which might suggest an important avenue of political exchanges that might affect SNTE's influence on education. In addition, the study suggests SNTE is far from a uniform block. Conflict and fragmentation inside the organization are signflcantly and negatively associated with student test scores. A single dissident section or multiple sections with opposing political inclinations in a state, are associated with lower average student test scores. The influence of the union over educational quality is a complex problem. This paper is presented only as an approximation with the intent offormalizing some of the potential channels through which this relationship becomes evident.