The variation of neutron importance with energy and position has been investigated in a multiregion critical assembly having a series of test regions typical of slightly epithermal to fast power reactors. Values of neutron importance at the center of the test regions were measured using neutron sources and a reactivity oscillator. The variation of neutron importance with position was determined using neutron sources in conjunction with a dynamic reactivity-measurement technique. Analysis of data from similar beryllium- and carbon-moderated test regions indicates the significance of the Be(n, 2n) reaction. The neutron sources used in this study were Po-Be, Po-B11, Po-CaF2, Po-Li7, Mock Fission and Sb-Be. Published source spectra were used in the analysis of the data; the Mock-Fission-source spectrum was determined by gamma-ray spectroscopy. Relative yields of these sources were determined by calibration in a manganese sulfate bath.