Abstract: The pattern of activity of aspartate aminotransferase enzyme (AST) following canine distalization stage in orthodontic treatment is being investigated. The enzyme is released to the gingival crevis following tooth movement resulted from focal necrosis in the adjacent periodontal ligament. The finding of enzymatic response could enhance our understanding of the enzyme`s role during tooth movement. This study investigated the potential of AST as a biological marker to monitor tooth movement by determining its activity in Gingival Crevicular Fluid (GCF) during bodily tooth movement (canine distalization). About 13 patients age between 14-26 years participated in the study. All patients had orthodontic treatment using fixed appliances. For every subject, one upper canine being the test tooth while contralateral canine served as control. Distalization force (100 g) was applied only to the test tooth and GCF was collected from the mesial and distal sites of test and control teeth every week until week 12 (week 0, 1, 4, 8 and 12). The activity of AST in the GCF was determined spectrophotometrically (at 30°C, 340 nm). The AST activity in the GCF of test teeth in all patients significantly increased at week 1 (p<0.05) compared to control. The activities during the following weeks (4-12) were similar to one another and stabilized but significantly higher than week 1 (p<0.05). There was 100% increment of AST activity from week 1 to 4. AST activity in the distal site was also significantly higher (p<0.05) than the mesial site of test teeth. There is no difference in AST activities when age group (adult and adolescent) were compared (p>0.05). The AST activity appears to be enhanced with the applied force and showed no differences between adult and adolescent patient. Therefore, AST has the potential as a biological marker to monitor progress of orthodontic tooth movement.