Fog is a defining feature of the coastal California redwoodforest and fog inputs via canopy drip in summer can constitute30% or more of the total water input each year. Agreat deal of occult precipitation (fog and light rain) isretained in redwood canopies, which have some of the largestleaf area indices known (Westman & Whittaker,Journalof Ecology63, 493–520, 1975). An investigation wascarried out to determine whether some fraction of interceptedfog water might be directly absorbed through leafsurfaces and if so, the importance of this to the water relationsphysiology of coast redwood,Sequoia sempervirens.An array of complimentary techniques were adopted todemonstrate that fog is absorbed directly byS. sempervirensfoliage. Xylem sap transport reversed direction duringheavy fog, with instantaneous flow rates in the direction ofthe soil peaking at approximately 5–7% of maximum transpirationrate. Isotopic analyses showed that up to 6% of aleaf’s water content could be traced to a previous night’sfog deposition, but this amount varied considerablydepending on the age and water status of the leaves. Oldleaves, which appear most able to absorb fog water wereable to absorb distilled water when fully submersed at anaverage rate of 0.90 mmol m2s-1, or about 80% of transpirationrates measured at the leaf level in the field.Sequoiasempervirenshas poor stomatal control in response to adrying atmosphere, with rates of water loss on very drynights up to 40% of midday summer values and rates above10% being extremely common. Owing to this profligatewater use behaviour ofS. sempervirens, it appears that foghas a greater role in suppressing water loss from leaves, andthereby ameliorating daily water stress, than in providingsupplemental water to foliar tissuesper se. Although directfoliar absorption from fog inputs represents only a smallfraction of the water used each day, fog’s in reducing transpirationand rehydrating leaf tissues during the most activegrowth periods in summer may allow for greater seasonal carbon fixation and thus contribute to the very fast growthrates and great size of this species.