Sphagnum has basic importance in the ecology of the wetlands of arctic and boreal regions. In addition to its ability to soak up large amounts of water, it can create an acid habitat by exchange of hydrogen ions for other cations in solution, such as Ca, Mg, K, and Na. It is thus able to make a habitat suitable to itself and other bog plants and to control the direction of bog development. Our study was undertaken to determine the correlation of Sphagnum species to pH and moisture gradients in hummock-hollow complexes. Our observation are limited to open mat communities in northern Michigan, near the Straits of Mackinac. The vertical sequence of species in hummock-hollow complexes investigated in northern Michigan begins with Sphagnum cuspidatum (and, atypically, S. majus) submerged in hollows. Sphagnum recurvum occurs at bases and lower sides of hummocks, while Sphagnum magellanicum is found on the sides, particularly on steeper faces above S. recurvum. S. capillaceum occurs on the upper sides and, on smaller hummocks, covers the top. On larger hummocks, S. capillaceum var. tenellum replaces the var. capillaceum, and the highest hummocks are crowned with S. fuscum. This sequence can be viewed as successional. A pH gradient exists between the hollow and the top of the hummock with a significant decrease in pH values as height above the water table increases. It is suggested that pH is dependent at least in part on the galacturonic acid content and cation-exchange capacity of individual species of Sphagnum and in part on amounts of water present in the system. The sequence is also related to a water gradient.