SUMMARYA single auxin was present in the sporangiophores and mycelium ofPhycomyces blakesleeanus, and was secreted into the synthetic culture medium containing asparagine as sole nitrogen source. It was found in the acid fraction, and was identified as indoleacetic acid by bioassay and chromatography. The amounts produced were at least 0.03μg/g fresh weight of sporangiophores, and 0.001-0.002μg/ml of culture medium. The Ehrlich and Salkowski reagents gave positive reactions for IAA only on some of the chromatograms although enough auxin was present to give clearly detectable spots. This erratic behavior, especially towards the Salkowski reagent, was probably due to interference by contaminating reducing compounds, which were indeed detected in the extracts. Preliminary evidence suggests the presence of indole-carboxylic acid in the sporangiophores. Unidentified Salkowski- and Ehrlich-positive substances were always found at high Rf in the acid and neutral fractions from the sporangiophores and medium, but were absent from the acid fraction of the mycelium.