The differential fast-neutron flux above 0.5 MeV at one spatial position in the High Flux Isotope Reactor was determined experimentally using several threshold activation detectors. The series expansion technique utilizing the concept of least squares was used to obtain an approximate solution to the set of integral equations which are defined by the experimentally determined activation data. Good agreement was achieved between the integrated flux (i.e., the differential flux integrated above 0.5 MeV) determined in this work and the integrated flux determined independently using one-dimensional, multigroup diffusion theory.