Lanolin emulsion paste incorporating indolylacetic acid in concentrations of 10,000, 1,000, 100, and 10 μg./ml when applied with a micromanipulator to one side of Phycomyces sporangiophores had no clearly marked action on subsequent growth Griseofulvin, a metabolic product of Penicillium janczewskii and certain other moulds, at a concentration of 1OO μg./ml., produced a local increase in the rate of extension of the wall at or near the region to which it was similarly applied, with consequent curvature from the normal erect growth habit. Previously proposed explanations of the regulation of sporangiophore growth, and of phototropism, in terms of the action of heteroauxin and auxin-α are criticized, and attention directed to the danger of assuming that substances active in regulating the growth of cellulose-walled times will have a similar action on chitin-walled fungal hyphae.