The U.S. Department of Health and Human Services has made the promotion of regular physical activity a national health objective, and experts believe that physical education can play a significant role in the promotion of physical activity. Feasible measurement tools to assess physical activity behavior, by physical educators, are lacking. One validated instrument is the System for Observing Fitness Instruction Time (SOFIT; McKenzie, Sallis & Nader, 1991). SOFIT’s physical activity data are collected using momentary time sampling (MTS) with a 20-second interval length and provide estimates of Moderate to Vigorous Physical Activity (MVPA). Whether variations in interval lengths would adversely affect the accuracy of the MVPA data has not been investigated. From a clinical perspective, if physical education teachers are to utilize MTS procedures for on-going assessment they will require longer time intervals to collect accurate MVPA data. This project sought to determine the accuracy of MVPA levels collected using modified activity level categories of SOFIT with varying momentary time sampling (MTS) lengths (i.e., 20s, 60s, 90s, 120s, 180s, and random) relative to those collected through duration recording (DR), allowing assessment to be better integrated with instruction. Data were collected from videotapes of elementary physical education classes. Pearson-product moment (PPM) correlation coefficients and Average Error (AE) were utilized to demonstrate concurrent validity of the varying interval lengths. Results demonstrated moderate-low to high correlations between the 20s, 60s, 90s, and random interval lengths and the DR tactic. The MTS procedure that created the least amount of AE across classes was the 20s variable followed by the 60s, random, and 90s variables. Keyword(s): assessment, physical activity