The root canals of 48 single-canal canine teeth were prepared by four different methods. The coronal lingual wall and root canal space were lowered to 2 mm below the buccal wall of the roots. The prepared root was placed into acrylic resin so that 12 mm of buccal root was exposed. A slowly increasing force was applied at the junction of the buccal wall and root canal space until fracture occurred. The force of fracture of the buccal root wall of each root was recorded and the results in each group were compared. All teeth fractured at the buccal wall/acrylic resin junction. Instrumentation of the root canals significantly weakened the roots. Obturation of the canals in conjunction with Roth's 801 sealer did not strengthen the roots. Obturation of the canals in conjunction with a glass ionomer sealer significantly strengthened the roots as compared with roots instrumented but not obturated and those obturated with gutta-percha and Roth's 801 Sealer.