A measurement of the thermal neutron extrapolation length from a semi-infinite slab of pure water has been made using a mock plane-thermal-neutron source. The result is δ = 0.35 ± 0.03 cm, in agreement with calculations, which range from about 0.30 to 0.35 cm, but disagreeing with results inferred from pulsed neutron source experiments in small geometries, which range from about 0.40 to 0.50 cm.