Purpose: To determine if sealing dentin caries with a resin-modified glass ionomer (RMGI) decreases the number of viable microorganisms and demineralization depth, using an experimental dentin caries model of biofilms of Streptococcus mutans (S. mutans). Materials and Methods: Dentin caries lesions were created over a 15-day period on bovine dentin slabs with biofilms of S. mutans UA159. Once lesions were formed, biofilms were removed and the slabs were randomly allocated to one of the treatment groups: group A, surface sealed with a RMGI; or group B, unsealed slabs (control). At 5, 10, and 15 days, slabs were longitudinally sectioned to analyze the number of infiltrating viable bacteria by culturing on agar plates and demineralization depth by cross-sectional Knoop microhardness. Results: Dentin lesions sealed with RMGI decreased bacterial counts from the infected dentin, both with respect to baseline counts and also relative to the unsealed control (p 0.05). Consistently, demineralization depth was lower for sealed lesions when compared with unsealed slabs (p 0.05) at all analyzed timepoints. Conclusions: Sealing dentin lesions with RMGI appears to halt lesion progression in this experimental model by reducing bacterial viability. Keywords: tooth demineralization, dental caries, microbial viability, glass-ionomer cements, resin-modified glass ionomer, Streptococcus mutans, root caries, aging