A user interface management system (UIMS) for extensible software environments must promote uniformity, and yet be both extensible and powerful. We describe the architecture of Chiron-0, a UIMS designed to meet the demands of a software environment. We discuss the key concepts underlying the design and how those concepts are realized in the implementation of a prototype. Our experiences with the prototype brought to light the successes of our approach as well as its limitations. We devised ways to circumvent some of these problems; others are influencing a redesign effort that is currently under way.