Saccharomyces cerevisiae S288C produced two laminarinases (1,3-fi-glucanases) which were separated by diethylaminoethyl-Sephadex column chromatography; one was an endo-1,3-fl-glucanase, and the other was an exo-1,3-fi-glucanase active not only on laminarin but also on pustulan (1,6-,B-glucan) and on pnitrophenyl-fl-D-glucoside. A mutant defective in the production of this last enzyme was isolated, and the mutation was named exbl-1. The selection procedure was based on the capacity of exo-1,3-,8-glucanase to hydrolyze synthetic glucosides. The level of endo-1,3-f.-glucanase in cell extracts of the mutant was normal, but the exo-1,3-,8-glucanase could not be detected by column chromatographic analysis of these extracts. The mutant phenotype, recessive in heterozygous diploids, was stable through successive meioses and showed a Mendelian segregation, indicating that the mutation affected a single gene, which was named EXBI. The lack of production of exo-1,3-fl-glucanase persisted through all the phases of growth, but growth itself was not impaired by the enzyme deficiency.The production of 1,3-,B-glucanases by many' species of yeast is well established. Among them there are exo-1,3-fi-glucanases (1,9,11,14,15,20,27), which hydrolyze the ,8-0-glucosidic linkages at the nonreducing end of the polymer chain, leading to the release of glucose. These are distinguishable from endo-1,3-fi-glucanases,