1. Experimental plots of mulberry-field were irrigated with ground-water in late spring, and the damages in the plant by frost were examined in comparison with those in control plots unirrigated. 2. Irrigation was begun on April 10, 1954, and was continued thereafter, water being applied into the furrows every three days, in the amount of 30mm in depth for each time. 3. The first test-case of severe frost came on Apri1 21, and the second on April 28. Each night previous to the attacks, forecasted meteorologically, irrigation was intensified, by way of precaution, the depth of water applied being increased to 60 mm. 4. As the results of experiments, the damages in the plots irrigated proved decidedly small compared with those in the control plot unirrigated. 5. In the irrigated plots, the damage was rather severer in higher or younger boughs than in lower ones, while in the unirrigated plot, the relation was found inverse. 6. The rates of injury either in the number of boughs or in that of leaves were considerably larger in the irrigated plots than in the unirrigated plots. 7. The average minimum air temperatures observed at 1m above the ground were 1.6°C. in the irrigated plots, and 0.3°in the unirrigated plot. The initial temperature of the ground water used for irrigation was nearly constant at ca. 15°C.
