Sodium glycerophosphate, disodium p-nitrophenyl phosphate, glucose-1-phosphate, glucose-6-phosphate, hexose diphosphate, fructose-6-phosphate, muscle adenylic acid, and yeast adenylic acid were used as substrates to determine whether one or more alkaline phosphatases were present in the developing down feather;Phosphatase was found in the nucleoli, chromatin granules, nuclear membrane and cytoplasm of the cells in the mesodermal pulp. In the epidermis, phosphatase was present in the nucleoli of the cells of the barbs and in the nucleus and cytoplasm of the cylinder cells. The pattern of activity was the same with all substrates except yeast adenylic acid, although the intensity of the reaction varied with the various substrates. With yeast adenylic acid, epidermal activity was confined primarily to the nucleoli of the cells;To determine whether the differences in reactivity were due to different phosphatases, the activating or inhibiting effects of Mg++, Mn++, Be++, Zn++, Fe+++ , HAsO4=, HAsO3=, CN-, semicarbazide, iodoacetate, Versene, glycine, alanine, cysteine, histidine, methionine, proline, valine, serine, lysine, and arginine were studied in conjunction with several of the substrates;Mg++ was necessary for activity with muscle adenylic acid, hexose diphosphate, and fructose-6-phosphate. At high concentrations, Zn ++ inhibited activity with all substrates except fructose-6-phosphate, and in low concentration stimulated activity with yeast adenylic acid. Yeast adenylic acid showed strong nucleolar activity at concentrations of arsenate which inhibited activity with all other substrates. Versene, iodoacetate, semicarbazide, Be++, and CN- in high concentrations inhibited phosphatase activity. Mn++, Fe+++, and HAsO3= had no apparent effect on phosphatase. Cysteine and histidine had a greater inhibitory effect on phosphatase than the other amino acids. At low concentrations, glycine promoted activity with muscle adenylic acid. Valine, proline, and methionine in low concentrations gave some indication of possible stimulation of activity with sodium glycerophosphate;The activity of phosphatase with yeast adenylic acid differs from that with muscle adenylic acid in its response to some of the inhibitors and activators. The activity with both of the adenylic acids differs from that obtained with sodium glycerophosphate. The fructose sugars were inhibited by lower concentrations of the chemicals than inhibited activity with sodium glycerophosphate;Epidermal phosphatase shows a greater sensitivity to the inhibiting agents than the phosphatase in the pulp. Cytoplasmic phosphatase is more easily inhibited than that in the nucleoli;These data indicate that the nucleolus is the source of at least part of the phosphatase. They also suggest that the developing down feather contains more than one phosphatase capable of utilizing the various substrates at a pH of 9.0--9.4.