Objective: To assess the sealing ability of ProRoot MTA when placed as an apical barrier using three different techniques. Method and Materials: Sixty freshly extracted single-rooted human teeth were decoronated and standardized to a root length of 15 mm. The root segments were prepared with Gates Glidden burs to simulate a divergent open apex of immature teeth and randomly assigned into three experimental groups of 20 samples each. MTA (5 mm) was placed by pluggers (Group A), paper points (Group B), or ultrasonic tips (Group C). A cotton pellet moistened with saline was placed in the root canals till the material set, and the coronal portion was sealed with Cavit. After 24 hours, the Cavit and cotton pellet were removed and the canals dried and obturated with warm guttapercha (Obtura) and AH 26 sealer. Coronal portions of all samples were then sealed with Cavit. All root segments were double-coated with nail varnish except for the open apex and were exposed to methylene blue dye for 48 hours at room temperature. The samples were sectioned longitudinally (layer thickness 0.4 mm) and the extent of dye penetration was measured with a stereomicroscope (magnification 20×). Results: The mean depth (± standard deviation) of dye leakage for Group A was 1.34 ± 0.5 mm, Group B 1.25 ± 0.4 mm, and Group C 1.14 ± 0.4 mm. There were no significant differences among the three experimental groups (.178 P .552). Conclusion: ProRoot MTA has a similar sealing ability when placed as an apical barrier with pluggers, paper points, or ultrasonic tips. Schlagwörter: apexification, mineral trioxide aggregate, sealing ability