The prompt neutron lifetime was calculated for the NBSR, a heavy water-cooled and -moderated research reactor at the National Institute of Standards and Technology. The method is based on the fact that the decay of a pulse of fast neutrons is related to the prompt neutron lifetime (and the multiplication constant for the reactor and the delayed neutron fraction). A Monte Carlo simulation of the decay is then used to calculate the prompt neutron lifetime at two points in the fuel cycle. At the start-up of a new cycle, the prompt neutron lifetime was calculated to be 774 ± 35 s, and at the end of a cycle, it was calculated to be 819 ± 48 s.