In plants, Glycoside Hydrolase (GH) Family 1 beta -glycosidases are believed to play important roles in many diverse processes including chemical defense against herbivory, lignification, hydrolysis of cell wall-derived oligosaccharides during germination, and control of active phytohormone levels. Completion of the Arabidopsis thaliana genome sequencing project has enabled us, for the first time, to determine the total number of Family 1 members in a higher plant. Reiterative database searches revealed a multigene family of 48 members that includes eight probable pseudogenes. Manual reannotation and analysis of the entire family were undertaken to rectify existing misannotations and identify phylogenetic relationships among family members. Forty-seven members (designated BGLU1 through BGLU47 ) share a common evolutionary origin and were subdivided into approximately 10 subfamilies based on phylogenetic analysis and consideration of intron-exon organizations. The forty-eighth member of this family ( At3g06510; sfr2 ) is a beta -glucosidase-like gene that belongs to a distinct lineage. Information pertaining to expression patterns and potential functions of Arabidopsis GH Family 1 members is presented. To determine the biological function of all family members, we intend to investigate the substrate specificity of each mature hydrolase after its heterologous expression in the Pichia pastoris expression system. To test the validity of this approach, the BGLU44 -encoded hydrolase was expressed in P. pastoris and purified to homogeneity. When tested against a wide range of natural and synthetic substrates, this enzyme showed a preference for beta -mannosides including 1,4- beta -D-mannooligosaccharides, suggesting that it may be involved in A. thaliana in degradation of mannans, galactomannans, or glucogalactomannans. Supporting this notion, BGLU44 shared high sequence identity and similar gene organization with tomato endosperm beta -mannosidase and barley seed beta -glucosidase/ beta -mannosidase BGQ60.