A STUDY TO DEVELOP INSTRUMENTS TO MEASURE CHILD BILINGUALISM AND BICULTURAL SOCIALIZATION WAS CONDUCTED IN DEL RIO, TEXAS, A MEXICAN-AMERICAN COMMUNITY IN WHICH SCHOOL IS TAUGHT IN BOTH SPANISH AND ENGLISH. THREE INSTRUMENTS WERE DEVELOPED--(1) A SERIES OF 6 TESTS FOR MEASURING LINGUISTIC COMPETENCE IN ENGLISH, (2) A SIMILAR SERIES OF 6 TESTS FOR SPANISH, AND (3) A SERIES OF 3 INSTRUMENTS FOR MEASURING SOCIALIZATION. TEST BATTERIES FOCUSED ON THE ORAL-AURAL USE OF LANGUAGE IN REALISTIC SCHOOL SITUATIONS. A RANDOM SAMPLE OF 97 FIRST GRADERS WAS GROUPED INTO 4 EXPERIMENTAL SECTIONS TAUGHT BILINGUALLY BY MEXICAN-AMERICAN TEACHERS AND INTO 4 CONTROL SECTIONS TAUGHT IN ENGLISH BY ENGLISH TEACHERS. THE CONTROL GROUP CHILDREN WERE GIVEN THE ENGLISH SERIES AND INVENTORY OF SOCIALIZATION WHILE THE EXPERIMENTAL CHILDREN WERE GIVEN BOTH THE ENGLISH AND SPANISH SERIES AND THE INVENTORY. ANALYSIS OF TEST RESULTS SHOWED THAT THE EXPERIMENTAL SUBJECTS WERE AS COMPETENT IN ENGLISH AS THOSE LEARNING ONLY IN ENGLISH AND ALSO BETTER ADJUSTED SOCIALLY. TEST INSTRUMENT VALIDITY AND RELIABILITY WAS DETERMINED AND AN ITEM ANALYSIS CARRIED OUT. APPENDIXES WHICH INCLUDE FACSIMILES OF TEST INSTRUMENTS AND ANALYSES OF EXPERIMENTAL DATA COMPRISE MORE THAN HALF OF THE REPORT. (MS)