We present an algorithm for automatic testing of distributed programs, such as Unix processes with inter-process communication, Web services, etc. Specifically, we assume that a program consists of a number of asynchronously executing concurrent processes or actors which may take data inputs and communicate using asynchronous messages. Because of the large numbers of possible data inputs as well as the asynchrony in the execution and communication, distributed programs exhibit very large numbers of potential behaviors. Our goal is two fold: to execute all reachable statements of a program, and to detect deadlock states. Specifically, our algorithm uses simultaneous concrete and symbolic execution, or concolic execution, to explore all distinct behaviors that may result from a program's execution given different data inputs and schedules. The key idea is as follows. We use the symbolic execution to generate data inputs that may lead to alternate behaviors. At the same time, we use the concrete execution to determine, at runtime, the partial order of events in the program's execution. This enables us to improve the efficiency of our algorithm by avoiding many tests which would result in equivalent behaviors. We describe our experience with a prototype tool that we have developed as a part of our Java program testing tool jCUTE.