In this research project, subject terms from the Dewey Decimal Classification (DDC) Schedules and Relative Index were incorporated into an online catalog as searcher's tools for subject access, browsing, and display. Four features of the DDC were employed to help searchers browse for and match their own subject terms with the online catalog's terminology: (1) subject terms in the DDC Schedules; (2) hierarchical arrays of related terms in the DDC Schedules; (3) subject terms in the DDC Relative Index; and (4) class numbers in the DDC Schedules and Relative Index. The effectiveness of the DDC in an online catalog was tested in online retrieval experiments with library patrons and staff at the Library of Congress, the New York State Library, the Public Library of Columbus and Franklin County (Ohio), and the Mathematics Library of the University of Illinois at Urbana-Champaign. The research findings chiefly address subject searchers' use of a library classification in the information retrieval environment of an online catalog. These findings provide insight into searchers' problems with online displays of the DDC Schedules and Relative Index, bibliographic record displays enhanced with subject information from the DDC, and online catalog capabilities necessary to support searching, displaying, and retrieving library cataloging and classification information. As a research project studying online catalog use, the DDC Online Project furthers general knowledge of library patron use of online catalogs, regardless of the extent to which these catalogs have been enhanced by a library classification. (Author/THC)