Objective: To compare the penetration and adaptation of a moisture tolerant resin-based sealant with a conventional resin-based sealant in different occlusal fissure types in vitro by scanning electron microscopic (SEM) technique. Methods: Fifty sound, intact extracted premolars and third molars were included. They were randomly and equally allocated to receive either Seal-Rite (conventional resin-based sealant) or Embrace WetBond (moisture-tolerant resin-based sealant). Etching and sealant applications were performed as per manufacturers' instructions. Specimens were sectioned at the deepest part of the fissure and viewed under SEM. Under SEM, fissure types were classified as U, V, I, IK or inverted Y. SEM images were analysed using Biowizard image analysis software to measure penetration and adaptation. The data were statistically tested. Results: U fissures showed the highest mean percentage penetration. V forms exhibited the best adaptation. IK forms showed the poorest percentage penetration and adaptation. Fissure form significantly affected adaptation but not penetration. Embrace WetBond penetrated better than Seal-Rite into all the fissures but adapted excellently only in U fissures. Conclusion: Fissure morphology significantly affected sealant adaptation. Moisture-tolerant Embrace WetBond was better than conventional Seal-Rite in penetration and adaptation into fissures. Keywords: occlusal fissure form, moisture-tolerant sealant, resin-based sealant