The possibility of using the silicon content of wheat as a measure of transpiration in the field has been investigated. Silicon is concentrated in the husks, containing about one-third of the total silicon in the plant, and so the husks were used for most measurements. Samples, from field trials extending over several seasons and from a number of localities, showed that the percentage silicon (% Si) in the husks is closely related to water transpired. The equation, water used(cm) = 7.4+4.6(%Si) (r = 0. 99 , n = 8), should be applicable to most of the wheat-growing areas of southern Australia, but the addition of fertilizers, particularly nitrogen, modifies the equation. The relationship between weight of silicon in the tops and water transpired is consistent with the known solubility of Si(OH)4 in soils, but the correlation is poorer than that using percentage silicon.