Accurately assessing physical activity behavior continues to challenge researchers. Many of the problems associated with doing so were featured at the "9th Measurement and Evaluation Symposium of the Measurement and Evaluation Council of the American Association for Active Lifestyles and Fitness." At the symposium the need for continued research was emphasized; including research using alternative methodologies. To date, the vast majority of physical activity assessment studies have relied on nomothetic research methods. Although nomothetic research has been responsible for numerous scientific advances, this approach may obscure rather than illuminate when there is a great deal of intra-individual variability among study participants. In these cases, an idiographic research approach may yield unique insights. In idiographic research, regularity is sought at the individual level rather than at the group level. This is achieved by collecting multiple samples of “data” from the same person (e.g., a case study, time-series research design). With this in mind, this study’s aim was to monitor a series of physical activity evaluations in one individual over a period of 42 months. The case, who resided in a major metropolis, was a Caucasian male, in his early 30's. Physical activity assessments were obtained in the following sequence: (A) 24 months using an uniaxial accelerometer; (B) 6 months using an electronic pedometer; and (C) 12 months using both devices concurrently. In each phase of the study, data (i.e., kilocalories of energy expenditure and/or number of steps taken) were recorded by the participant on either a weekly (A and B) or daily (C) basis. Data were analyzed using STATISTICA™ software. Analyses revealed significant intra-individual variability in the case’s physical activity behavior across years, months, and days. Specifically, in A, the case was significantly less active in year 1 as compared to year 2. The “most active” month during year 1 was August and in year 2, July. In B, a correlation of .44 was observed between the case’s accelerometer and pedometer scores; and, in C, according to the case’s pedometer scores, the case was “more active” on Fridays in comparison to Sundays, with no differences detected by the accelerometer. In this study there was a substantial degree of intra-individual variability observed in a single case’s physical activity behavior, thus further highlighting the difficulties researchers may encounter when trying to accurately assess physical activity behavior using nomothetic research approaches. Continued idiographic studies may prove useful. Keyword(s): assessment, measurement/evaluation, physical activity