The advantages of accelerometers includes their small size and the ability to store data continuously over long periods of time. The newest commercially available accelerometer, the RT3 Triaxial Research Tracker (Stayhealthy, Inc.), is based on the technology of the TriTrac R3D. The main purpose of this study was to compare the accuracy of the RT3 Research Tracker to the Tritrac R3D. Both instruments collect data in activity movement counts in three planes [mediolateral (X), anteroposterior (Y), and vertical (Z)] and vector magnitude (VM) [a composite of the counts]. A secondary purpose of this study was to devise cut off points for RT3 VM counts related to physical activity intensities (MET values). Twelve men [age 23 + 2 yr, ht=177.3 + 7.4 cm, wt=72.4 + 9.9 kg, VO_2max=61.5 + 9.6 ml·kg^-1·min^-1, HR_max=189 + 6 b·min^-1] and thirteen women [age 24 + 2 yr, ht=165.9 + 7.4 cm, wt=60.0 + 8.2 kg, VO_2max=48.5 + 6.8 ml·kg^-1·min^-1, HR_max=189 + 8 b·min^-1] volunteered to participate in this study. Subjects performed six minutes each of the following exercise conditions on a motorized treadmill: slow walking (4.8 km·h^-1), fast walking (6.4 km·h^-1), and jogging (9.7 km·h^-1) at 0% grade; and fast walking (6.4 km·h^-1) at 5%, 10%, and 15% grades. The subjects then participated in slow walking (4.8 km·h^-1), fast walking (6.4 km·h^-1), and jogging (9.7 km·h^-1) on an outdoor field. Paired t-tests found the RT3 had higher VM than the R3D for all exercise levels (p < .001). ANOVA found the RT3 VM counts were significantly different with changes in speed both on the treadmill [F(2,48)=541.0, p< 0.001] and outside [F(2,48)=472.1, p< 0.001], as well as changes in treadmill grade [F(2,48)=13.2, p< 0.001]. Tukey post hoc tests identified that increases in speed and grade were detected by increases in average VM counts. Vector magnitude count cut offs corresponding to MET levels were identified for the RT3 as Light [MET < 3, (VM < 1237)], Moderate [MET 3 – 5.99, (VM 1237 - 3245)], Hard [MET 6 – 8.99, (VM 3246 - 5250)], Very Hard [MET > 8.99, (VM > 5260)].