A STUDY WAS UNDERTAKEN TO INVESTIGATE FOREIGN LANGUAGE LEARNING IN BOTH INTRAINING AND FIELD PROGRAMS OF THE PEACE CORPS, AND TO DETERMINE INDIVIDUAL AND EXPERIENCE FACTORS AFFECTING THE RATE OF LANGUAGE LEARNING. THE OBJECTIVE WAS TO DEVELOP A PROTOTYPE FOR A PARAMETRIC STUDY INVOLVING SUCH FACTORS AS LANGUAGE APTITUDE AND PRIOR LANGUAGE TRAINING. A TOTAL OF 444 TRAINEES IN SPANISH AND 51 TRAINEES IN PORTUGUESE AT THE UNIVERSITY OF NEW MEXICO WERE GIVEN PLACEMENT TESTS AND LANGUAGE APTITUDE AND PROFICIENCY TESTS AT VARIOUS STAGES OF THE TRAINING PROGRAM. RESULTS SHOWED THAT PRIOR KNOWLEDGE, NOT APTITUDE, DETERMINED THE DEGREE OF FLUENCY. A FOLLOWUP STUDY WAS DONE IN THE FIELD WITH SIMILAR RESULTS. RECOMMENDATIONS WERE MADE RESPECTING (1) LENGTH OF COURSE, (2) FURTHER TRAINING IN THE FIELD, (3) THE IMPORTANCE OF VARIABLES SUCH AS APTITUDE AND PRIOR KNOWLEDGE IN TRAINEE SELECTION, AND (4) DEVELOPMENT OF A PROGRAM OF LANGUAGE PROFICIENCY MEASUREMENT. (KL)