The growth of mushroom A. campestris was studied with the aim of obtaining a clear description of the mechanics of the process. First, by means of measuring mushrooms marked with carmine dots at regular intervals, the details of the location of the growth region during different phases of growth were described. From paraffin sections it was possible to show that, following an early period of randomly oriented hyphae, the hyphae became oriented longitudinally in the growth zone and that from approximately the 18 mm stage onward there is probably little or no increase in cell number (at least in the stalk) for the increase in cell size is sufficient to account for the total increase in size. The fact that increase in size is primarily by cell elongation in the stalk was supported by the weight-size relationships. This elongation of the cells was then shown to involve a transport of materials from the mycelium in the soil since the dry weight increased proportionately with the wet weight. In conclusion, it was pointed out that all the major problems of mapping out the overall shape of the mushroom and the laying down of the majority of the cells are done at a very early stage and that the subsequent three- to four-fold increase in size is merely the expansion of specific parts of this preformed bud, which involves the sucking up of nutrient materials from the soil mycelium.