  We have performed electrical resistivity measurements of a polycrystalline
sample of FeSe$_{0.25}$Te$_{0.75}$, which exhibits superconductivity at $T_{\rm
c} \sim 14$ K, in magnetic fields up to 55 T to determine the upper critical
field $\mu_{0}H_{\rm c2}$. In this compound, very large slopes of
$\mu_{0}H_{\rm c2}$ at the onset, the mid-point, the zero-resistivity
temperatures on superconductivity are determined to be -13.7, -10.1, and -6.9
T/K, respectively. The observed $\mu_{0}H_{\rm c2}(T)$s of this compound are
considerably smaller than those expected from the Werthamer-Helfand-Hohenberg
model, manifesting the Pauli limiting behavior. These results suggest that this
compound has a large Maki parameter, but it is smaller than that calculated for
a weak-coupling superconductor, indicating a large superconducting gap of this
compound as a strong-coupling superconductor.
