Purpose: Recently, highly flowable resin fillers have been proposed for filling root canals using a single-cone approach. The purpose of this study was to test the hypothesis that polymerization shrinkage, which is inherent to this type of filler, can break the close initial contact between the main core and the surrounding dentin even if root dentin infiltration has occurred. Materials and Methods: Twenty roots of anterior teeth were prepared chemomechanically and divided into 4 groups of 5 samples each. Groups 1 and 2: hydrophilic resin filler injected and spread, roots sectioned after setting; group 3: hydrophilic resin filler injected and spread, dental substrate dissolved after setting; group 4: hydrophobic resin sealer lentulo-spiraled and spread, roots sectioned after setting (as control). In groups 1, 2, and 3 the material was used following manufacturer's instructions. The section surfaces of groups 1 and 4, and the resin components of group 3 were processed for conventional FE-SEM. The section surfaces of group 2 were observed successively under environmental and high-vacuum conditions. Results: Microscopic examination of the resin-dentin interfaces of groups 1 and 2 showed the existence of resin-dentin interdiffusion zones (RDIZ); however, the close initial contact between the main core and the surrounding dentin was often lost. In group 3, resin tag morphology was well characterized. Identical morphological features were observed in the resin-dentin interfaces in group 4. Conclusion: Since the existence of resin tags did not exclude the existence of a gap between the main core and the adjacent RDIZ, our results confirmed the proposed hypothesis. Keywords: adaptation, filling, infiltration, root dentin, seal