OVER A 13-YEAR PERIOD, EXTENSIVE LONGITUDINAL DATA WERE COLLECTED ON EVERY ASPECT OF THE LINGUISTIC BEHAVIOR OF 338 OAKLAND, CALIFORNIA, SCHOOL CHILDREN AS THEY PROGRESSED FROM KINDERGARTEN THROUGH GRADE 12. TAPED ORAL INTERVIEWS AND A WIDE RANGE OF TESTS AND INVENTORIES WERE CONDUCTED ANNUALLY TO MEASURE READING ACHIEVEMENT, LISTENING ABILITY, SKILL WITH WRITTEN LANGUAGE, AND ABILITY AND GROWTH IN FLUENCY WITH ORAL LANGUAGE. RESULTS INDICATE THAT STUDENTS WHO DEVELOP PROFICIENCY IN SPOKEN LANGUAGE WILL ALSO DEVELOP SKILLS IN WRITING, READING, AND LISTENING. UNLIKE STUDENTS OF LESSER ABILITY, STUDENTS WITH HIGH LANGUAGE PROFICIENCY WILL (1) USE MORE VARIED AND FLEXIBLE SYNTAX, (2) USE RELATIONAL WORDS EARLIER, MORE OFTEN, AND MORE ACCURATELY, (3) EXPRESS TENTATIVENESS AND SUPPOSITION MORE FREQUENTLY, (4) EMPLOY MORE ADVERBIAL CLAUSES OF CAUSE, CONCESSION, AND CONDITION, AND (5) USE MORE ACCURATE AND OPTIONAL GRAMMATICAL TRANSFORMATIONS IN THEIR SENTENCE STRUCTURES. CORRELATIONS SHOW THAT STUDENTS FROM ABOVE-AVERAGE SOCIOECONOMIC GROUPS WILL USE NON-STANDARD ENGLISH LESS FREQUENTLY AND WILL DEVELOP LANGUAGE SKILLS EARLIER AND TO A GREATER COMPETENCY THAN THOSE FROM BELOW-AVERAGE SOCIOECONOMIC GROUPS. (DL)