The differential effects of language proficiency on standardized test performance of elementary school students (grades 4 through 6 are examined for the San Diego (California) Unified School District. Ethnicity is examined as a mediating variable circumscribing the relationship between language proficiency and test performance. Test performance is based on the Comprehensive Tests of Basic Skills scores in reading, language, and mathematics for approximately 21,000 students. Using a multiple regression model, the relative effects of language proficiency among the different racial and ethnic groups are analyzed. Ethnic groups considered are: (1) African Americans; (2) Asians; (3) Filipinos; (4) Hispanics; (5) Native-Americans (includes Alaskan Indians); (6) Pacific Islanders; (7) Southeast Asians; and (8) Whites. The results suggest that performance is strongly related to ethnicity. Reading is the area most sensitive to acculturation, while language is more likely to measure accomplishment based on memorization, and consequently effort. Mathematics is the area least dependent on English language competence. Student language status is identified as a critical factor in test performance. This study does not document the effects of traditional socioeconomic factors. Three tables and one flowchart present study data. A nine-item list of references is included. (SLD)