This volume contains the proceedings of the ninth international workshop on logic-based program synthesis and transformation (LOPSTR 99) which was held in Venice (Italy), September 22-24, 1999. LOPSTRistheannualworkshopandforumforresearchersinthelogic-based program development stream of computational logic. The main focus used to be on synthesis and transformation of logic programs, but the workshop is open to contributions on logic-based program development in any paradigm. Previous workshops were held in Manchester, UK (1991, 1992), Louvain-la-Neuve, B- gium (1993), Pisa, Italy (1994),...