Abstract The capability to conduct automated radiation-transport simulations of delayed-gamma emission spectra at discrete (line) energies created by the products of neutron fission and activation has been developed for MCNPX. To do so, the CINDER'90 isotopic transmutation code has been merged into MCNPX to seamlessly supply time-dependent, decay-chain atom densities for 3400 nuclides. A new dataset containing ENDF/B-VI emission-probability line data for 979 nuclides has been created for MCNPX, with the balance of the 3400 nuclides treated using existing 25-group emission spectra. Cumulative distribution sampling functions have been developed to accommodate line and multigroup emission data. Fission-product sampling for fissions induced by sub-20-MeV neutrons uses fission-yield data for thermal ( E < 1 eV), fission-spectrum (1 eV ≤ E < 14 MeV), and high-energy ( E ≥ 14 MeV) neutrons for isotopes of uranium, plutonium, thorium, americium, californium, curium, einsteinium, fermium, and neptunium. For higher-energy neutrons, LAHET, a physics package that is also a part of MCNPX, generates a list of residual nuclides. In Part II, we present simulation results for models based on experiments conducted by Fisher and Engle (1964) and Beddingfield and Cecil (1998) to validate the new capability. As will be seen therein, the MCNPX results are in good agreement with the measured data. Finally, in Part III we augment the Monte Carlo presentation with a transport-theory formulation to provide a succinct encapsulation of the relevant physics. The new MCNPX delayed-gamma development offers a powerful new tool for fission-related signature recognition.