The purpose of this in vitro study was to examine the effect of various powder-liquid ratios of an experimental resin-modified glass-ionomer polymer cement on dye penetration when the cement was used as a pit and fissure sealant. Eighty extracted human third molars were randomly assigned to one of four pit and fissure sealant conditions; a resin-based sealant or one of three variations in powder-liquid rations (1.4:1.0, 1.8:1.0, and 2.0:1.0) of an experimental, light-activated, resin-modified glass-ionomer cement. After sealant placement the teeth were thermocycled and immersed in methylene blue dye. The teeth were sectioned, and the extent of the dye penetration along the sealant-enamel interface was measured linearly. The resin-sealed occlusal fissures showed statistically significantly less dye penetration than did the three powder-liquid ratios of the experimental resin-modified glass-ionomer-cement sealant.