1. It has been shown that the direct counting method for soil protozoa devised by Kopeloff and Coleman for use in liquid media gives results entirely comparable with those obtained by a dilution method.

2. The factors governing the relation between the protozoa and the soil particles are those of surface action, and the capacity of various substances, sand, soil and clay, for retaining these organisms is specific and constant.

3. Coarse sand is capable of withdrawing per gram approximately 145,000 amoebae and flagellates per c.c. from a suspension of any strength. Fine sand withdraws approximately 980,000 per c.c: soil and partially sterilised soil 1,650,000, ignited soil 1,500,000 and clay 2,450,000.

4. These figures are constant for given material and organisms and are independent of the concentration of the suspension, the time of action, or whether the suspension contains cysts or active forms of the amoebae and flagellates investigated. Also the action is the same when the experiment is performed with a suspension of living or dead organisms.

5. Experiments with the ciliate—Colpoda cucullus—show that coarse sand per gram retains 27,000 per c.c.; fine sand per gram 185,000 per c.c; soil and partially sterilised soil 280,000 per c.c; ignited soil 270,000 per c.c. and clay 450,000 per c.c.