The wilty tomato mutant flacca and the normal variety Rheinlands Ruhm were compared in terms of: (1) potassium transport into and out of the guard cells, (2) cell wall properties which include protein, hydroxyproline and peroxidase activity, and (3) activity of indol-3yl-acetic acid oxidase. Also studied were the effects of auxin on stomatal behaviour and peroxidase activity when applied to normal plants during development, and the short-term effect of abscisic acid on the resistance of flacca stomata to closure under plasmolysis. Potassium transport, wall protein and hydroxyproline all seemed to be equal in mutant and normal plants. Peroxidase activity was higher in the soluble and wall fractions of the mutant, and decreased toward normal in the mutant treated with abscisic acid. More stomata were open and peroxidase activity was higher in normal plants treated with auxin during development. The percentage of open stomata under plasmolysis was lower and their aperture size was smaller in the epidermal strips taken from abscisic-acid-treated mutant plants than from control mutant plants.