The AAECC conferences focus on the algebraic aspects of modern computer science, which includes the most up-to-date and advanced topics. The topic of error-correcting codes is one where theory and implementation are unifiedinto a subject both of mathematical beauty and of practical importance. Algebraic algorithms are not only interesting theoretically but also important in computer and communication engineering and many other fields. This volume contains the proceedings of the 9th AAECC conference, held in New Orleans, LA, in October 1991. Researchers from Europe, America, Japan and other regions of the world presented papers at the conference. The papers present new results of recent theoretical and application-oriented research in the field.