This paper discusses the design and evaluation of an artificial agent for collaborative musical free improvisation. The agent provides a means to investigate the underpinnings of improvisational interaction. In connection with this general goal, the system is also used here to explore the implementation of a collaborative musical agent using a specific robotics architecture, Subsumption. The architecture of the system is explained, and its evaluation in an empirical study with expert improvisors is discussed.A follow-up study using a second iteration of the system is also presented. The system design and connected studies bring together Subsumption robotics, ecological psychology, and musical improvisation, and contribute to an empirical grounding of an ecological theory of improvisation.Designers of human-computer interactive systems for musical improvisation have taken diverse approaches to system development. Some of these systems implement an abstract model of the human mind (e.g., Rowe 1992), while others implement a model of the sonic and emotional organizational principles of improvisation, and emergent