Iowa fens have three distinct, concentric vegetation zones: a border zone, a sedge mat zone, and a discharge zone where ground water supplying the fens comes to the surface. Calamagrostis inexpansa Gray, Viola nephrophylla Greene, Scirpus americanus Pers. and Carex spp. are the dominants in the border zone. The composition of this zone is quite variable both within and between fens. The sedge mat is composed primarily of Rhynchospora capillacea Torr. Other species normally found in this zone are Lobelia kalmii L., Muhlenbergia racemosa ( Michx.) BSP, Parnassia glauca Raf.,Triglochin maritima L. and Scirpus americanus. Although all of the species in this zone are also found in the other zones, this zone is readily distinguishable because of the low stature of the vegetation. The discharge zone is dominated usually by Carex spp. or Calamagrostis inexpansa, except at one fen where Phragmites communis Trin. and Helianthus grosseserratus Martens are the dominants. The sedge mat zone on the average has fewer species per quadrat ( 5.8 versus 6.0 and 8.6), lower Shannon-Weaver index ( 0.8 versus 1.15 and 1.40) and higher Simpson's index ( 0.58 versus 0.42 and 0.32) than the discharge and border zones, respectively.