Light-adapted sporangiophores of the fungus Phycomyces respond to sudden darkening by a temporary decrease in the rate of elongation, after a latent period of several minutes. The reaction time of this "dark growth" response is compound like that of the "light growth" response. It is, moreover, shorter the more intense the previous illumination. The rate of dark adaptation following adaptation to a very large range of light intensities is found to be proportional to the logarithm of the preceding light intensity. It is shown that a constant amount of dark adaptation takes place before the response occurs. On the assumption that changes in the rate of growth reflect changes in the concentration of a substance which at constant light intensity is in equilibrium with a light-sensitive material, possible equations for such a photostationary state are examined. The most reasonable formulation requires that the partial velocity of the "light" reaction be taken proportional to log I instead of to I directly.