Epiphytic microorganisms present on cotton plants synthesized 3-indoleacetic acid (IAA) from tryptophan. Microorganisms from the root zone synthesized 3 times the amount of IAA when compared with the shoot zone and the root zone contained a much higher number of microorganisms. IAA-synthesizing activity was eliminated when the tissues were treated with a weak solution of mercuric chloride. Various tests on the possible accumulation of IAA from external sources showed that IAA synthesized outside the plant does not accumulate in the plant. Although epiphytic microorganisms synthesize IAA in large amounts, they do not influence the IAA content of the plant due to (1) lack of available tryptophan, (2) destruction of the auxin by the microflora, and (3) the polar movement of the auxin.