The relation of moisture potential to the germination of wheat was studied by supplying water to the seeds in the vapour phase, at controlled relative humidities. An apparatus for this purpose and a method of attaining the necessary precise control of temperature are described. The critical level of moisture potential at which germination is completely inhibited was not reached. Even when, as a result of infection, the duration of the experiments was limited to 20 days, up to 20 per cent, of the seeds germinated at a potential of −320 metres of water or a pF of 4.5. This is considerably drier than Permanent Wilting Point, which is represented by a potential of −160 metres of water, or pF 4.2. There is a marked effect of moisture potential on the time taken to germinate, as times of emergence varied from 2 to over 20 days over a range of potentials of from zero to −320 metres of water, but it is probable that very little further germination would have occurred after 15 days had not fungal infection enabled the seeds to obtain liquid water by bridging the air-gap between seed and water source. This infection is probably responsible for the reduction in viability of the seeds at the lower potentials, as there is an indication that under conditions where germination is postponed considerably, seeds exhibit a reduced germinating capacity probably due to an increased susceptibility to infection. The failure of some individual seeds to germinate at potentials which allow other seeds to germinate is not due to failure to attain a critical moisture content. In soil, the percentage germination at low potentials is similar to that observed without soil, but the reduction in the total germinating capacity, or viability, after 20 days is not as great, and this is probably due to the reduced incidence of visible infection observed under these conditions.