It has been shown theoretically and, in the case of 5 soils, exptlly. that a method involving the scattering and slowing down of fast neutrons by H may be used for measuring soil moisture. The method rests primarily on 2 considerations: Practically speaking, H is the only material which will slow up fast neutrons; and H in soils is present almost entirely in the form of water. In applying the above concepts, a fast-neutron source and a slow-neutron counter (source and counter combined into a single unit) are lowered into a small auger hole in the soil. The counting rate of neutrons which have been slowed by soil H is a measure of the moisture content. Based on the theory and the tests, the following conclusions are drawn: The method appears valid from oven-dryness to water-saturation, yields the moisture content per unit volume directly (not moisture content per unit oven-dry wt.), requires but a single calibration for all mineral soils (but the calibration depends on the depth of sample and other geometrical factors), appears to be independent of soil temp., texture, composition, compaction and concn. of soil soln, and should be particularly useful in measuring soil moisture in the field in situ. The above statement applies to mineral soils. Organic soils present a special problem which is discussed. Design of the portable apparatus for field measurements awaits development.