Effective control of the root rot fungus, Armillaria mellea, with methyl bromide requires diffusion of adequate dosages to depths as deep as the roots of the dead and diseased plants. Factors that govern diffusion to these depths are soil texture, porosity as it relates to moisture, amount and method of application, and the soil covers used. Diffusion patterns were determined by gas chromatographic analysis of soil atmospheres. A crop of sudangrass dried the soils sufficiently in one season to depths of 8 feet to enable methyl bromide to diffuse readily. Best downward movement was obtained by placing methyl bromide about 3 feet deep; downward movement was greater than lateral, and adequate dosages were obtained 10 to 12 feet deep. Increased dosage increased the range of effective fumigation. A plastic cover was required to give control at the surface and top 1 foot of soil. Polyethylene is a relatively poor cover compared to other plastics. The soils studied were coarse sandy Ioams and the fine-textured silt and sandy clay loams. Some results of these studies have been applied to commercial fumigations.