The function of the casing layer in the cultivation of the commercial white mushroom, Agaricus bisporus, was examined. The study was divided into 2 parts. Firstly, physical and chemical properties of 9 different casing materials were investigated and the findings related to the results of a cropping trial. Several materials, including granulated bark and substitutes structure air-filled affecting bark and mixtures of . 'Fibre-mix' and granulated 'Fibre-mix' and peat, proved to be suitable for the traditionally used peat casing. The of the casing material, especially the volume of pores, was found to be an important factor the fruiting of A. bisporus. The volume of water held by a casing material bore no relation to yield. Secondly, biological aspects of the casing layer were examined. Bacteria, identified as Pseudomonas Putida, were isolated from the casing layer and found to stimulate primordia formation and mycelial growth, but not strand development. Methods allowing the in vitro production of primordia were examined. A modification of Peerally's (1979) method which ensured low carbon-dioxide concentrations within the 'growth chambers' during sporophore initiation proved successful. Good evidence was found for the existence of a plasmid(s) in one of the bacterial isolates. The plasmid(s) appeared to be involved in the process of sporophore initiation and development in A. bisporus and also carried genes for mercury resistance.