Streptococcus mutans (S. mutans) is the main etiological agent of dental caries, and adheres to the tooth surface through the sortase A (SrtA)-mediated cell wall-anchored protein Pac. Inhibition of SrtA activity results in a marked reduction in the adhesion potential of S. mutans, and the frequency of dental caries. Morin is a natural plant extract that was previously reported to inhibit Staphylococcus aureus SrtA activity. Here, we demonstrate that morin has an inhibitory effect against S. mutans UA159 SrtA, with an IC50 of 27.2 +/- A 2.6 mu M. Western blotting demonstrated that 30 mu M morin induced the partial release of the Pac protein into the supernatant. The biofilm mass of S. mutans was reduced in the presence of 30 mu M morin, which was not caused by a decrease in S. mutans viability. These results indicate that morin might be important as a new agent to prevent caries.