A class of UDP-glycosyltransferases (UGTs) defined by the presence of a C-terminal consensus sequence is found throughout the plant and animal kingdoms. Whereas mammalian enzymes use UDP-glucuronic acid, the plant enzymes typically use UDP-glucose in the transfer reactions. A diverse array of aglycones can be glucosylated by these UGTs. In plants, the aglycones include plant hormones, secondary metabolites involved in stress and defense responses, and xenobiotics such as herbicides. Glycosylation is known to regulate many properties of the aglycones such as their bioactivity, their solubility, and their transport properties within the cell and throughout the plant. As a means of providing a framework to start to understand the substrate specificities and structure-function relationships of plant UGTs, we have now applied a molecular phylogenetic analysis to the multigene family of 99 UGT sequences in Arabidopsis. We have determined the overall organization and evolutionary relationships among individual members with a surprisingly high degree of confidence. Through constructing a composite phylogenetic tree that also includes all of the additional plant UGTs with known catalytic activities, we can start to predict both the evolutionary history and substrate specificities of new sequences as they are identified. The tree already suggests that while the activities of some subgroups of the UGT family are highly conserved among different plant species, others subgroups shift substrate specificity with relative ease.