The applicability of the zero-neutron-lifetime approximation in describing the effects of neutron-level fluctuations is investigated for reactivities near and above prompt critical. It is concluded that meaningful statistical information can be obtained by the zero-lifetime model above prompt critical, and an approximate procedure for joining this model to a deterministic finite-lifetime model is suggested. Illustrative examples, comparing numerical results obtained by this approximation with more accurate finite-lifetime statistical calculations, are presented. In addition, application is made to Los Alamos and Livermore superprompt-critical burst experiments which fall outside of the practical computing range of the finite-lifetime model described in Part II. It is found that the agreement of calculation and experiment is as good as was found previously for a set of subprompt-critical burst experiments.