This study examined the percentage time estimates of momentary time sampling against the real time obtained with handheld computers in a natural setting. Twenty-two concurrent observations were conducted in elementary schools by one observer who used 15-s momentary time sampling and a second who used a handheld computer. Results for the six behaviors showed a close correspondence between the momentary time sampling percentage observation intervals and the real time percentage observation time, although 15-s momentary time sampling tended not to sample low-frequency short-duration behaviors. The results confirmed laboratory findings that short-interval momentary time sampling estimates percentage time accurately for a wide range of behavior frequencies and durations, and suggested that observers using momentary time sampling in a natural setting are able to obtain accurate data.