The enzyme O-acetylserine sulfhydrylase participates in the biosynthesis of l-cysteine in bacteria and plants. The structure of isoenzyme B (CysM) from Escherichia coli was established in a hexagonal crystal form at 2.7 A resolution (wild-type) and in a merohedrally twinned tetragonal crystal form at 2.1 A resolution (surface mutant). Structural superpositions revealed the variations with respect to isoenzyme A (CysK) and explained the different substrate specificities. A geometric model of the reaction catalyzed by CysM is proposed. Both isoenzymes are used for the production of l-amino acid derivatives as building blocks for the synthesis of peptides and peptidomimetic drugs. Since the structure of CysM revealed a remarkable main chain variation at the active center, it constitutes a further starting point for engineering mutants with novel substrate specificities.