The transport mean free path of thermal neutrons in heavy water is determined from measurements of the neutron density in heavy water (99.4 atoms of deuterium per 100 atoms of hydrogen element) at various distances from a cadmium plate. In the region investigated, the density is found to be a linear function of the distance from the cadmium plate. When this straight line is extrapolated, the density vanishes at a distance d = 1.64 cm. behind the plate. On the basis of transport theory it is known that d = 0.71 lt, where lt is the transport mean free path in the medium. The final result is lt = 2.4 cm. in pure heavy water. The measurements of the neutron density show that it falls below the straight line as the cadmium surface is very closely approached, in agreement with transport theory.