Objective: Streptococcus mutans produces 3 distinct ghtcosyhransferases (GTase): GTase-1, GTase-SL and GTase-S. These enzymes synthesize from sucrose water-soluble and insohtble glucan that act cooperatively in mediating S. mutans adherence to tooth surface and dental plaque formation. This study investigated the relationship between the sucrose-dependent adherence and GTase-l expression level. Methods: In this study a Streptococcns-Escherichia eoli shuitter vector carrying GTase-I gene, pZBI, was constracted. The gene pZBI was then transformed into S. mutans GTase-I-deficient muttant B29, Two types of GTase-I expressing strain were obtained: GTase-I expression strain similar to the parent and a GTase-I overe.overexpression strain. Results: Sucrose-dependent adherence of these mutants was closely related to the amount of initial bacterial inoculum in the experiment, that is a small inoculum resulted in firm adherence and vice versa. The plasntid was reiso/ated from the GTase-I overexpressing strain. Nucleotide sequencing revealed that no mutation was found upstream of the GTase-I gene when compared with pZB1. Conclusion: The balance in the quantity of GTase-I and GTase-SI may play an important role in the adherence of S. matans in the presence of sucrose. (CJDR 1998; I:26-42)