THE APPREHENSION DATA SYSTEM, PART OF MIGRATION CONTROL STATISTICS IS MOST USEFUL IN STUDYING THE NUMBERS, CHARACTERISTICS, AND LABOR MARKET ROLES OF THE NATION'S ILLEGAL IMMIGRANTS. DATA GATHERED BY THE INS ON APPREHENSIONS REFLECT THE CHARACTERISTICS OF THOSE CAPTURED AND THE KINDS OF INS LAW ENFORCEMENT ACTIVITIES INVOLVED. THE SAME SOURCE PROVIDES INFORMATION ON THE NUMBER OF TIMES THAT DEPORTABLE ALIENS ARE LOCATED, THE NUMBER OF APPREHENSIONS FOR EACH OF SEVERAL DIFFERENT ENFORCEMENT ACTIVITIES, AND THE AMOUNT OF PERSONNEL RESOURCES ALLOCATED TO VARIOUS AREAS AND ENFORCEMENT ACTIVITIES. OTHER MIGRATION CONTROL STATISTICS WHICH ARE USEFUL IN THE STUDY OF ILLEGAL ALIENS INCLUDE ENTRY AND ADMISSIONS DATA, ENTRY DENIAL DATA, DEPARTURES OF ILLEGAL ALIENS, VISAS ISSUED AND DENIED, AND SYSTEMS OF OTHER AGENCIES. THE PRINCIPAL UTILITY OF THESE SYSTEMS COMES WHEN THEY ARE COMBINED TO BUILD AN INDEX OF ILLEGAL IMMIGRATION WHICH CAN BE USED TO MEASURE CHANGES OVER TIME. SPECIFIC LABOR MARKET DATA OBTAINABLE FROM APPREHENSION STATISTICS INCLUDE THE PERCENTAGE OF WORKING APPREHENDEES, THE PERCENTAGE OF ILLEGAL AGRICULTURAL WORKERS, THE AMOUNT OF INS RESOURCES EXPENDED ON LABOR MARKET INTERVENTION, AND THE COST OF REMOVING AN ILLEGAL IMMIGRANT WORKER FROM HIS JOB. FOUR POPULATIONS ARE SCREENED REGULARLY FOR ILLEGAL IMMIGRANTS: ENTRIES, ALIENS QUESTIONED BY THE BORDER PATROL, ALIENS SEEKING WELFARE BENEFITS IN LOS ANGELES COUNTY, AND ALIENS IN CONTACT WITH LOCAL LAW ENFORCEMENT AGENCIES. ANALYSIS OF THE APPREHENSION AND OTHER MIGRATION CONTROL STATISTICS REVEALS A MUCH HIGHER INCREASE IN THE RATE OF ILLEGAL ALIEN ENTRANCE TO THE U.S. THAN OF LEGAL ALIEN ADMISSIONS. PRODUCTIVITY INDICES FOR FOUR ENFORCEMENT CATEGORIES DISPLAY A DROP IN OFFICER HOURS PER APPREHENSION DESPITE STAFF INCREASES, SUGGESTING GROWTH IN BOTH STOCK AND FLOW OF ILLEGAL IMMIGRANTS. RESOURCE ALLOCATIONS SHOW THAT INS IS PAYING MORE ATTENTION TO WOULD-BE ILLEGAL IMMIGRANTS THAN TO ILLEGAL IMMIGRANT WORKERS. A TOTAL OF SIX TABLES AND SIX EXHIBITS ARE INCLUDED.