An apparatus is described for investigating theinsitucreep behavior of materials during irradiation with neutrons having a spectrum similar to that of a fusion power reactor. Techniques are discussed for remote, high‐vacuum creep testing during irradiation with high‐energy neutrons from a planar Be(d,n) source used on the 1.93‐m Isochronous Cyclotron at the University of California at Davis. Significant effects of radiation damage on the creep of niobium at a flux of 2×1016n⋅m−2 s−1are reported.