This article is devoted to the run-time redistribution of arrays that are distributed in a blockcyclic fashion over a multidimensional processor grid. While previous studies have concentrated on e ciently generating the communication messages to be exchanged by the processorsinvolved in the redistribution, we focus on the scheduling of those messages: how to organize the message exchanges into structured" communication steps that minimize contention. We build upon results of Walker and Otto, who solved a particular instance of the problem, and we derive an optimal scheduling for the most general case, namely, moving from a CYCLICr distribution on a P-processor grid to a CYCLICs distribution on a Q-processor grid, for arbitrary values of the redistribution parameters P, Q, r, and s. This work was supported in part by the National Science Foundation Grant No. ASC-9005933; by the Defense Advanced Research Projects Agency under contract DAAH04-95-1-0077, administered by the Arm...