AbstractWe present a parallel implementation of the three‐dimensional alternating direction implicit finite‐difference time‐domain (ADI‐FDTD) method in Cartesian coordinates using the message passing interface (MPI) library. Parallel implementations not only speed up computations but also increase the maximum solvable problem size. The application of the ADI scheme results in freeing the time‐step size from the Courant‐Friedrichs‐Lewy stability constraint. We demonstrate a speedup of the ADI‐FDTD method through parallelization, even though the communication overhead between the processors is proportional to the volume of the domain, rather than just the surface area of the subdomains as for standard parallel FDTD. We benchmarked our code on an IBM p690 symmetric multiprocessor and also a distributed memory computer cluster with a high‐bandwidth Infiniband interconnect. In both cases, satisfactory scalability and efficiency was demonstrated for simulation domains comprising up to 8 billion mesh cells. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1298–1304, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24310