Fast neutrons are produced inside a water moderator by a Po-α-Be source used effectively as a pulsed neutron source. The slowing-down time to the Cd edge is measured with a BF3 proportional counter used with a cadmium difference technique. The mean slowing down time to O.35 ev, and the variance thereof, are found to be 5.2 µsec and 8.0 µsec2, respectively. It is shown that this is in reasonable agreement with the results of a Monte Carlo calculation, provided collisions are considered to involve whole H2O molecules when the neutron energy falls below about 1.6 ev.