THIS PAPER IS A CASE STUDY OF THE ADOPTION BY GEOSCIENCE ABSTRACTS OF UNIDEK, A COMPUTER-COMPILED SYSTEMATIC SUBJECT INDEX BASED ON THE UNIVERSAL DECIMAL CLASSIFICATION (UDC). EVENTS LEADING TO A DECISION TO ADOPT THE SYSTEM, SOME THEORY OF INDEXES, PROBLEMS INVOLVED IN CONVERSION, AND SOME OF THE RESULTS ACHIEVED ARE REVIEWED. UNIDEK MAKES POSSIBLE THE MECHANICAL ANALYSIS OF COMPLETE UDC NUMBERS AND PROVIDES AN INDEX BY UDC NUMBERS, UDC HEADINGS, ARTICLE TITLES, AND ABSTRACT NUMBERS OF ARTICLES CITED. THE GREATEST WEAKNESS IN COMPUTER-AIDED INDEXING BY UNIDEK WAS FOUND TO BE THE LACK OF A COMPLETE MECHANICALLY SEARCHABLE FILE OF ALL UDC NUMBERS. MORE USE OF UDC SHOULD PROVIDE BETTER UDC SCHEDULES AND THEREFORE BETTER INDEXES. APPENDED IS AN ILLUSTRATION OF HOW A UNIDEK INDEX IS PREPARED AND USED, THE UNIDEK SYSTEM BLOCK DIAGRAM, AND SAMPLE PAGES FROM A UNIDEK INDEX. AN EARLIER VERSION OF THIS PAPER WAS PRESENTED TO THE 1965 FID CONGRESS (31ST, WASHINGTON, D.C., OCTOBER 7-16, 1965). THIS DOCUMENT IS AVAILABLE AS PB 175 926 FROM THE CLEARINGHOUSE FOR FEDERAL SCIENTIFIC AND TECHNICAL INFORMATION, SPRINGFIELD, VIRGINIA 22151, $3.00 FOR HARD COPY, $0.65 FOR MICROFICHE, 31 PAGES. (AUTHOR/JB).