We present an open and extensible architecture, ImAge, for content-based image retrieval in a distributed environment. The architecture proposes the use of system components with standard public interfaces for implementing retrieval functionality. The standardization of the components and their encapsulation in autonomous software agents result in functional stratification and easy extensibility. Collaboration of the independent retrieval resources in ImAge results in enhanced system capability. Reuse of existing retrieval resources is achieved by encapsulating them in agents with standard interfaces. The addition of independent agents with domain knowledege adds the capability of processing conceptual queries, while reusing the existing system components for feature-based retrieval. A communication protocol allows the declaration of the capabilities of the system components and negotiations for optimal resource selection for solving a retrieval problem. The use of mobile agents alleviates network bottlenecks. This paper describes a prototype implementation that validates the architecture. © Springer-Verlag 2000.