The IOGENE project at the University of North Texas Libraries applied user-centered design principles to redesign the interface to a unique digital library of cultural heritage materials, the Portal to Texas History. Since its launch in 2004, the interface had become dated and implementation of new functionality was constrained by the underlying technical infrastructure. Genealogists, a significant and under-studied class of digital library users, participated in the redesign of the Portal's interface. At the outset of the project, focus group discussions provided insights regarding genealogists' information needs as well as their research practices in relation to online information systems. In large part, these insights informed the functional requirements for the redesign of the Portal's user interface. Subsequent to each of two public releases of the redesigned interface, genealogists were engaged in usability testing. An online survey measured user satisfaction prior to and after the new interface was released. Results determined that satisfaction with the Portal significantly improved after the final release of the redesigned interface. The project's process and findings will be of interest to archives and digital libraries facing similar challenges in regard to redesigning their user interface and involving users in the design process.