THE RELATIVE EFFECTIVENESS OF TWO TYPES OF MECHANICAL MODELS FOR TEACHING ELEMENTARY SCHOOL STUDENTS TO USE THE PARTICLE IDEA OF MATTER TO EXPLAIN CERTAIN PHYSICAL PHENOMENA WAS INVESTIGATED. SUBJECTS WERE RANDOMLY SELECTED FROM STUDENTS ENROLLED IN GRADES TWO THROUGH SIX IN A SCHOOL SYSTEM. A SERIES OF DEMONSTRATIONS AND RELATED QUESTIONS WERE USED TO SUBDIVIDE THE POPULATION INTO THOSE WHO DID AND THOSE WHO DID NOT USE THEORETICAL MECHANICAL MODELS IN EXPLAINING PHYSICAL PHENOMENA. STUDENTS WERE THEN RANDOMLY ASSIGNED TO TREATMENT GROUPS. ONE GROUP RECEIVED INSTRUCTION THROUGH USE OF A DYNAMIC MODEL, A SECOND GROUP RECEIVED INSTRUCTION INVOLVING A STATIC MODEL, AND A CONTROL GROUP RECEIVED NO INSTRUCTION. ALL SUBJECTS IN EXPERIMENTAL GROUPS WERE TESTED, INSTRUCTED, AND RETESTED ON AN INDIVIDUAL BASIS. APPLICATION OF ANALYSIS OF VARIANCE TO DATA REVEALED THAT (1) THERE WAS A SIGNIFICANT DIFFERENCE BETWEEN THE SCORES OF STUDENTS IN THE EXPERIMENTAL GROUPS AND THOSE IN THE CONTROL GROUP, (2) SCORES OF STUDENTS IN THE GROUP THAT USED MODELS IN EXPLAINING PHYSICAL PHENOMENA WERE SIGNIFICANTLY HIGHER THAN THOSE OF THE NON-MODEL STUDENTS, AND (3) THERE WERE NO SIGNIFICANT DIFFERENCES IN ACHIEVEMENT BETWEEN GRADE LEVELS. THE DYNAMIC MODEL WAS NOT SUPERIOR TO THE STATIC MODEL IN TEACHING STUDENTS TO USE MODELS IN EXPLAINING PHYSICAL PHENOMENA. (AG)