Abstract : Bibliographic databases (including databases based on open access) are routinely used for bibliometric research. The value of a specific database depends to a large extent on the coverage of the discipline(s) under study. A number of studies have determined the coverage of databases in specific disciplines focusing on inter-disciplinary differences. However, little is known about the potential existence of intra-disciplinary differences in database coverage. Focusing on intra-disciplinary differences, the paper documents large database coverage differences within two disciplines (economics and psychology). The point extends to include both the uneven coverage of specialties and research traditions. The implications for bibliometric research are discussed, and precautions which need to be taken are outlined.