In samples of soil taken 3 weeks after treatment with dilute formalin (1:500 to 1:1000), Trichoderma viride had established itself as the dominant fungus, and this dominance persisted during the 6 months period of sampling. On agar, T. viride was more tolerant of formalin than were seven other common soil fungi. T. viride also became dominant after treatment of soil with carbon disulphide, at the rate of 0·5 ml./250 g. moist soil. After higher dosages (1 to 10 ml. carbon disulphide), the dominant fungi were two Ascomycetes, Penicillium luteum and Aspergillus fischerii, which were found to be highly tolerant of carbon disulphide in vitro, by comparison with other fungi tested. Recolonization of soil sterilized, by intermittent steaming, from an inoculum of unsterilized soil was followed by means of the ‘soil recolonization tube’, which comprises an 18 in. length of tube with seven side-arms at 2 in. intervals. From the side-arms, which were protected from outside contamination by metal caps, micro-samples of soil were taken at intervals for the making of soil plates, and in this way the progress of soil colonization by fungi along the length of the main tube could be followed. In closely packed soil, the first fungal colonizers were fast-growing Phycomycetes, but in loosely packed soil, their place was usually taken by Trichoderma viride. Soil recolonization tubes were also used to study the movement of formalin and carbon disulphide, respectively, through the soil. Distribution of the fumigant, which was injected at the central side-arm, was determined after an interval by taking samples of soil for plating from the side-arms on either side of it.