The purpose of this study was to examine agreement between the Walk4Life DUO pedometer (W4L; Walk4Life, Plainfield, Illinois, USA) and two criterion instruments in the measurement of physical activity. Participants (N = 189, M = 16.74 years, SD = 0.99) in high school physical education concurrently wore the DUO (i.e., comparison instrument) and Yamax SW651 pedometers (Yamax Corp., Japan; i.e., criterion instrument), and were video recorded for off-site systematic behavioral observation (i.e., criterion instrument). Physical activity measures were steps/minute (i.e., DUO and SW651) and physical activity time (minutes; i.e., DUO and behavioral observation). Relative agreement between physical activity measures was strong (r greater than or equal to 0.97, p less than or equal to 0.05). However, overall absolute agreement suggests that the DUO pedometer underestimates steps/minute (mean difference score [M[subscript diff]] = 3.13, SD = 4.73, t = 9.07, p = 0.0001), overestimates physical activity time (M[subscript diff] = -10.76, SD = 6.46, t = -17.22, p = 0.0001), and does not produce clinically acceptable steps/minute (mean absolute percent error [M[subscript APE]] = 10.82%, SD = 9.82) and physical activity time (M[subscript APE] = 76.13%, SD = 37.04) outcomes. In conclusion, it is recommended that researchers and practitioners cautiously interpret DUO steps/minutes and not employ the DUO physical activity time function during physical activity programs, such as high school physical education, to estimate physical activity time. (Contains 2 tables and 4 figures.)