A COMPREHENSIVE LABORATORY INVESTIGATION CONCERNING THE PHYSICAL PROPERTIES OF STRUCTURAL LIGHTWEIGHT CONCRETES CONTAINING FOUR COMMERCIALLY AVAILABLE FLY ASH AGGREGATES IS PRESENTED. AIR-ENTRAINING AGENT REQUIREMENTS WERE HIGH WHEN FLY ASH AGGREGATE FINES WERE USED. HOWEVER, WHEN THESE FINES WERE COMPLETELY REPLACED WITH NATURAL SAND, THE AIR-ENTRAINING AGENT REQUIREMENTS WERE NORMAL. DUE TO THE GRADING OF THE FLY ASH AGGREGATE FINES, CONCRETES CONTAINING ONLY COARSE AND FINE FLY ASH AGGREGATE WERE VERY HARSH AND DIFFICULT TO CAST. HOWEVER, CONCRETES CONTAINING NATURAL SAND USED AS A PARTIAL REPLACEMENT OF FLY ASH AGGREGATE FINES WERE QUITE WORKABLE. SINCE NATURAL SAND HAD TO BE USED IN ALL CONCRETES, THE 28-DAY UNIT WEIGHTS WERE LIMITED TO A MINIMUM OF ABOUT 96 LB PER CU FT AND INCREASED TO A MAXIMUM OF ABOUT 116 LB PER CU FT DEPENDING UPON THE AMOUNT OF NATURAL SAND FINES AND PORTLAND CEMENT USED. THE MEASURED PHYSICAL PROPERTIES OF THESE FOUR CONCRETES FALL WITHIN THE RANGES SHOWN IN THE "GUIDE FOR STRUCTURAL LIGHTWEIGHT AGGREGATE CONCRETE" OF THE AMERICAN CONCRETE INSTITUTE. /AUTHOR/