Parameter measurements in a 1.3% enriched UO2 lattice with H:U = 0.42 have been performed. These measurements are an extension of an experimental program in the TRX critical facility of the Bettis Atomic Power Laboratory. Earlier measurements were made for a wide range of water-to-uranium (H2O:U) volume ratios (1:1 to 8:1) using 4-ft (1.2-m)-high slightly enriched, 0.387-in. (0.98-cm)-diam uranium metal or oxide fuel rods clad with aluminum. The new data have been compared with current analytic techniques, using both P-1 and P-3 multigroup analysis in the epithermal neutron energy range and Monte Carlo multigroup methods for thermal neutrons. This extremely undermoderated lattice provides a very stringent test for both the computational methods and the neutron cross sections used. The quantities measured were: the ratio of epithermal-to-thermal radiative captures in U238 (ρ28); the ratio of captures in U238 to fissions in U235 (the modified conversion ratio, CR*); the ratio of U238 fisions to U235 fissions (δ28); and the ratio of epithermal-to-thermal U235 fissions (δ25). In addition, activations were obtained with thermal-neutron detectors of widely different spectral response. The results indicate that the calculational methods predict the parameters very well, except for δ28. The discrepancy in δ28 may be due to inadequate U238 inelastic scattering cross sections, but this conclusion requires additional study. Monte Carlo calculations of thermal-neutron detector activations show that use of either the Nelkin or Koppel kernel gives results that agree with the data.