As well as for their protective effect against sports injuries, interest is increasing in the value of mouth-guards for augmenting muscle strength. In this study the influence of two types of mouth-guards -commercial mouth-guards (FMG) and custom-made mouth-guards (CMG) - on grip power, back strength,peak torque in isokinetic knee extension, peak torque in isokinetic knee flexion and peak pedaling powerin rugby players are examined. The results are as follows. 1. No augmentation effect for grip power or back strength were noted in either of FMG and CMG. 2. A significant augmentation effect for peak torque in isokinetic knee extension was observed in CMG.However, neither FMG and CMG showed any affect on peak torque in isokinetic knee flexion. 3. A maximal pedaling power augmentation effect was noted in CMG but not in FMG.4. From the above-mentioned results, it was concluded that CMG had an augmentation effect on themuscle strength displayed in slow speed isokinetic exercises.