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The Actigraph (AG; model 7164 v. 2.2) accelerometer is widely accepted as a valid means of physical activity (PA) assessment. However, no studies have investigated interinstrument reliability for data collection in a free-living condition.To determine agreement (on raw outputs and derived variables) between AG accelerometers worn concurrently on the right hip (RH) and left hip (LH) during 24 h of free-living.Ten distinct pairings of AG were evaluated on 10 participants (four males, six females; age = 30.1 ± 3.8 yr). AG were worn during waking hours (other than water activities). Raw outputs were activity counts and steps. Derived variables were time (minutes) in sedentary, light, moderate, vigorous, and MVPA (moderate + vigorous) intensity using published cut points. Intraclass correlations (ICC), absolute percent error (APE), coefficient of variation (CV), and paired t-tests were evaluated between accelerometers.Raw variables of total activity counts and steps, and derived variables of time in sedentary, light, vigorous, and MVPA intensity displayed high interinstrument reliability. The ICC, APE, and CV values for moderate intensity were 0.98, 18.8%, and 13.5% and, for MVPA, 0.99, 3.7%, and 4.9%, respectively. Only MVPA displayed a statistically significant mean difference (2 min) between RH versus LH AG, but the effect size was small (ES = 0.07).Results display evidence of decreased interinstrument reliability of moderate-intensity time assessed during free-living. If separate moderate and vigorous categories are not necessary for specific research questions, MVPA may be the optimal intensity range to report, at least in terms of interinstrument reliability.