The AAECC conferences focus on the algebraic aspects ofmodern computer science, which includes the most up-to-dateand advanced topics. The topic of error-correcting codes isone where theory and implementation are unifiedinto asubject both of mathematical beauty and of practicalimportance. Algebraic algorithms are not only interestingtheoretically but also important in computer andcommunication engineering and many other fields. This volume contains the proceedings of the 9th AAECCconference, held in New Orleans, LA, in October 1991. Researchers from Europe, America, Japan and other regions ofthe world presented papers at the conference. The paperspresent new results of recent theoretical andapplication-oriented research in the field.