ALMOST NO EXPERIMENTAL EVIDENCE AVAILABLE SUBSTANTIATES THE CLAIM THAT THE BEST POSSIBLE WAY OF TEACHING SCIENCE IS BY BASING THE PRESENTATION UPON THE STRUCTURE OF A SCIENCE. RESEARCH IN THIS AREA HAS BEEN CLOUDED BY THE FAILURE TO CLEARLY SPECIFY THE ORDERED SEQUENCE OF MATERIALS AND INADEQUATE TESTS TO DETERMINE--(1) IF THE DESIRED ORDER EXISTS, AND (2) IF RANDOM ARRANGEMENT OF MATERIALS PRODUCES A DIFFERENT ORDER. IN THE STUDY RELATED HERE, THERE WERE THREE OBJECTIVES. THE FIRST OBJECTIVE WAS TO PRODUCE A SET OF PROGRAMMED MATERIALS WRITTEN TO CONFORM TO A DEFINITION OF ORDER, AND AN ALTERED VERSION OF THESE MATERIALS. THE SECOND OBJECTIVE WAS TO DEVISE MEANS OF DETERMINING IF--(1) THE TWO VERSIONS EFFECTIVELY TAUGHT THE SUBSTANTIVE CONTENT, AND (2) THE STRUCTURED UNIT WAS TRULY STRUCTURED AND DIFFERED FROM THE UNSTRUCTURED UNIT. THE THIRD AIM WAS TO INSTITUTE AN EXPERIMENT WHICH WOULD PROVIDE DATA FOR--(1) MEASURING THE EFFECTIVENESS OF THE TWO UNITS, (2) DECIDING IF THE UNITS WERE STRUCTURED OR NOT, AND (3) EXAMINING SOME EFFECTS OF STRUCTURE ON MEASURES OF TRANSFER AND ACHIEVEMENT. APPROXIMATELY 200 FOURTH, FIFTH, AND SIXTH GRADERS SERVED AS SUBJECTS. THERE IS A FULL DISCUSSION OF THE RESULTS AND OF THE POTENTIAL WORTH OF THE MODELS USED FOR MEASURING THE EFFECTIVENESS AND EXTENT OF STRUCTURE IN THE MATERIALS. THIS PAPER WAS PRESENTED AT THE AMERICAN EDUCATIONAL RESEARCH ASSOCIATION CONVENTION, CHICAGO, ILLINOIS, FEBRUARY, 1968. (IM)