Sterilization brings about significant changes in the physical and chemical, as well as biological properties of soil which thus becomes a different and often more favourable medium for microbial activity. In an investigation of a root rot of greenhouse tomatoes marked differences in numbers of fungi, bacteria, and actinomycetes were noted in both soils and rhizospheres as a result of sterilization with steam, chloropicrin, and formaldehyde. Roots invariably supported much higher numbers of the three groups of organisms studied, thus displaying the common "rhizosphere effect". Numbers of bacteria were considerably greater on infected than on healthy roots.Qualitative differences in fungi and bacteria were also noted in both soils and rhizospheres. Of particular interest was the tendency for bacteria with simple food requirements and those stimulated by amino acids to predominate in the rhizosphere, and for those with more complex nutritional needs to predominate in soils apart from the roots.It is suggested that such nutritional investigations of rhizospheres may be useful in studies on the physiological activity of plant roots.