A bog mat surveyed in 1934, and at irregular intervals since, was reexamined in 1967. After 33 years the position of the margin of the mat was essentially unchanged, but the width of the zones had changed. The Decodon on the aquatic margin had increased slightly, and the larch-shrub zone had expanded outward, greatly reducing the sedge mat. The indications are that the concentric zones surrounding open water expand and contract under changing conditions, principally fluctuating water levels.