We describe the design and implementation of an adaptive and extensible graphical debugger for metacomputing applications. This "metadebugger" has been developed in the context of the Harness framework, a dynamic and reconfigurable software infrastructure for distributed computing. A major distinguishing feature of this metadebugger is its support for dynamic addition and deletion of nodes in the distributed computing framework at runtime. The metadebugger also includes several other novel features, including a "step into" mechanism for remote method invocations (RMI) that unifies the debugging mechanism for local and remote calls. In addition, an online visualization tool, a navigation tool, a breakpoint manager, and a simple script language interpreter are integrated into the metadebugger. The metadebugger is also capable of invoking third-party debuggers that might implement other useful debugging facilities in heterogeneous computing environments.