Amoebae (Vahlkampfia) and ciliates (Colpoda steini) exist in the soil in an active state. After the center of petri dishes containing sterile soil have been inoculated with cultures of protozoa mixed with Bact. radicicola these organisms diffuse from the place of inoculation to the periphery. Detection of protozoa some time afterward at a considerable distance from the place of inoculation shows that they are living in the soil and can move freely therein. These species of protozoa and Bact. radicicola were found to multiply rapidly in the soil used, as shown by the following: introduction of 1 loop of culture into sterile soil is sufficient to inoculate, within a few days, the whole volume of soil in the dish with protozoa; the smallest quantity of soil taken from any part of the dish would then give a rich culture of protozoa on the agar plate within 2-3 days. The activity and rate of diffusion of bacteria and protozoa in the soil depend first of all on moisture and mechanical structure. At a moisture content of 15-20% (25-40% of the water-holding capacity) bacteria and amoebae are active, but diffuse slowly in the soil, whereas Colpoda steini, in most cases, does not diffuse in the soil farther than 1 cm. from place of inoculation. At a soil moisture content of more than 20% all protozoa are extremely active and diffuse rapidly through the whole soil. When the moisture content of the soil is comparatively small, amoebae diffuse more rapidly than ciliates, but at a greater moisture content (more than 60% of the water-holding capacity) the infusoria move more rapidly than the amoebae. Bacteria, in their progressive movement, always precede the protozoa, the bacteria (Bact. radicicola) serving as a source of food for the protozoa.