The diurnal course of water potential, stomatal conductance and transpiration was measured on mature umbels (the major evaporating surface) of papyrus (Cyperus papyrus L.) growing in a fringing swamp on Lake Naivasha, Kenya.Umbel water potential declined only slightly during the morning but fell rapidly after midday to a minimum value of-1.5 M Pa in early afternoon. The two main structures forming the umbels, the bracteoles and rays, showed similar patterns of change of stomatal conductance throughout the day. The values of conductance indicate major stomatal opening during the morning, partial midday closure and some recovery of opening during the afternoon.It appears that the increase in water vapour pressure deficit of the air is the major cause of the midday closure of the stomata and that plant water potential has little effect. The reason why transpiration is reduced at high vapour pressure deficits when water is freely available to the roots is not clear. However, it is speculated that the restricted water movement into the plant from the anaerobic root environment has the effect of reducing the uptake of toxic ferrous iron.The daily total of canopy transpiration is estimated to be 12.5 mm, twice the value previously reported for papyrus but similar to daily valus determined for other wetland communities.