The construction and testing of a new differential transpiration porometer for stomatal measurements are described. The principle involved is explained, and the mathematical theory is dealt with in an appendix. As a result of experiments comparing the new instrument's readings with those obtained with a Wheatstone bridge porometer, a relation between diffusive conductance and viscous flow conductance is found; in the case of wheat stomata this approaches a cubic relation and in the case of Talinum triangulare a quadratic one.