We report a study in which we examined the gait modulations performed by a group of healthy adults, naïve to the purpose of the study, while walking across a curb under natural environmental conditions out of doors. Data were collected for step time and step length using a videocamera and a twenty metre walkway. Location of foot positions during each walk was achieved using a novel method to account for parallax and out of plane motion. The results obtained indicated that during both curb ascent and descent, subjects were able to predict accurately the need to make an adjustment to step length in order to avoid placing the foot in a potentially hazardous position on or near the curb. These step adjustments involved both lengthening and shortening the step as well as keeping it unchanged. In any case, the selection of strategy was not apparently influenced by the walking speed but was a function of the predicted foot position with respect to the curb. Also, the time at which the step adjustment was made appeared independent of the time to contact with the curb. These results confirm some findings obtained under laboratory conditions, but also suggest that, under natural circumstances, accurate planning for obstacle negotiation can be made some distance before the obstacle.