Young children's expected social behaviors develop within particular cultural contexts and contribute to their academic experience in large part through their relationships with their teachers. Commonly used measures focus on children's problem behaviors, developed from psychopathology traditions, and rarely situate normative and positive behaviors in context. Building from the literature on parenting goals and socialization expectations in Latino families and a preliminary ethnographic study ("Proyecto Educando Ninos"), we constructed a survey in English and Spanish that measures the expected social behaviors of Mexican-heritage children, ages 3-6 years, using parent and teacher reports. Use of the BEAR Assessment System facilitated the refinement of this instrument to assess the socialization of young, Mexican American children. We report on the psychometric properties of the Mexican American Socialization (MAS) Scale, utilizing item-response theory. Analyses indicate the MAS Scale is reliable and an ecologically valid indicator of multiple constructs of the expected social behaviors of young Mexican American children. In particular, the 41-item "Bien Educado" subscale showed good reliability and is consistent with socialization constructs described in the literature. Implications for research and practice are discussed. (Contains 4 tables and 2 figures.)