The thermal neutron flux in a water medium from a source disk containing water irradiated in the Materials Testing Reactor has been measured using a BF3 detector. Neutrons from N17 and photoneutrons from N16 gamma rays have been observed. The radial distribution of thermal neutrons from an isotropic point source of N17 neutrons in a water medium out to 35 cm has been deduced, and has been compared with a theoretical calculation of the distribution based on Holte's values of the collision density at 1 ev from a point isotropic source of 1-Mev neutrons. From the results of this experiment and Holte's distribution, the square of the diffusion length plus the age from 1 ev to thermal energies, has been calculated to be 8.2 cm2. This value is in reasonable agreement with previously reported measured values.