This paper presents artificial immune system for optimal scheduling of thermal plants in coordination with fixed head hydro units. Numerical results of two test systems have been presented to demonstrate the performance of the proposed algorithm. The results obtained from the proposed algorithm are compared with those obtained from differential evolution, particle swarm optimization and evolutionary programming technique. From numerical results, it is found that the proposed artificial immune system based approach is able to provide better solution than differential evolution, particle swarm optimization and evolutionary programming in terms of minimum cost and computation time. (C) 2010 Elsevier Ltd. All rights reserved.