Frenching of tobacco is a non-infectious, or physiological, disease characterized by reticular chlorosis of the leaves, cessation of stem and branch elongation, strap-leaf formation, and greatly increased leaf number. These symptoms develop usually in the order given. The roots appear to be normal except for slight stunting and thickening. Recovery may occur spontaneously or when the plant is transplanted to a normal soil.Natural isoleucine can be used tQ duplicate these symptoms in aseptic culture, water culture, and soil (8). Necessary concentrations range from about 10 p.p.m. up, depending on the type of culture and variety of tobacco.Frenching in the field does not seem to be associated with the presence of isoleucine in soil (11). The accumulation of free isoleucine in the tobacco plant and the hormone-like morphological responses caused thereby appear, therefore, to result from functioning of an internal receptor mechanism only.