Recent experimental findings support theoretical predictions that across walking conditions the motor system chooses foot placement to achieve a constant minimum "margin of stability" (MOSmin) - distance between the extrapolated center of mass and base of support. For example, while step width varies, similar average MOSminexists between overground and treadmill walking and between overground and compliant/irregular surface walking. However, predictions regarding the invariance of MOSminto step-by-step changes in foot placement cannot be verified by average values. The purpose of this study was to determine average changes in, and the sensitivity of MOSminto varying step widths during two walking tasks. Eight young subjects walked on a dual-belt treadmill before and after receiving information that stepping on the physical gap between the belts causes no adverse effects. Information decreased step width by 17% (p = .01), whereas MOSminwas unaffected (p = .12). Regardless of information, subject-specific regressions between step-by-step values of step width and MOSminexplained, on average, only 5% of the shared variance (β = 0.11 ± 0.05). Thus, MOSminappears to be insensitive to changing step width. Accordingly, during treadmill walking, step width is chosen to maintain MOSmin. If MOSminremains insensitive to step width across other dynamic tasks, then assessing an individual's stability while performing theses tasks could help describe the health of the motor system. © 2012 Human Kinetics, Inc.