A method and apparatus for isolating portions of multi-tasking software for integration testing (debugging) are disclosed. One or more applications software tasks can advantageously be isolated for testing from other "remote" tasks in the same subsystem or other subsystems, or one or more complete computer subsystems having multi-tasking software can be isolated from other "remote" subsystems having other multi-tasking software. Such isolation facilitates debugging without having to modify the application software. In accordance with illustrated embodiments of the invention, tasks or complete subsystems are isolated using a test task and an external user. The test task is operable to alter communications pathways for the transfer of messages between tasks that are supported by a common communications mechanism that is resident in multi-tasking computer systems. The external user is operable to emulate the functions of the remote software tasks. A message from a task being tested that is intended for a remote task in the same or a different subsystem is rerouted to the test task and then to the external user. The external user emulates the functions of the remote task, generates a response message, and delivers the response message, via the test task, to an appropriate task under test. Thus, without any modification to the software tasks, such tasks can be debugged for integration with other tasks in the system. As such, the final production software is identical to the debugged software.