The KRE6 gene product is required for synthesis of the major beta-glucans of the yeast cell wall, as mutations in this gene confer reduced levels of both the (1----6)- and (1----3)-beta-D-glucan polymers. Cloning and sequencing of KRE6 reveals a gene encoding a predicted 80-kDa protein with a central transmembrane domain and the topology of a type II membrane protein. Null mutants of KRE6 grow slowly, have larger cells, and show a reduction in alkali-insoluble wall glucans. The mutants show good viability and are not osmotically sensitive, but they are more susceptible to beta-glucanase digestion and mechanical stress than wild-type cells. The specific activity of the GTP-dependent, membrane-associated, in vitro (1----3)-beta-glucan synthase is reduced 50% in kre6 null mutants, and this reduction correlates with the mutation in meiotic tetrads. Transformants of kre6 null mutants with a KRE6 gene expressed from a centomere-based vector show a 4- to 5-fold increase in in vitro (1----3)-beta-glucan synthase activity over transformants with the vector alone. The phenotype and structure of the KRE6 product, Kre6p, suggest that Kre6p may be a beta-glucan synthase, and if so, it implies that beta-glucan synthases are functionally redundant in yeast. Alternatively, Kre6p may be part of a single multiprotein glucan synthase or modulate its activity. Use of KRE6 should permit a genetic analysis of eukaryotic (1----3)-beta-glucan synthesis.