The resistivity has been measured as a function of temperature in magnetic fields up to 12 T for a typical Tl₂Ca₂Ba₂Cu₃O_x sample. The resistive transition width has been observed to broaden with increasing field while the transition onset remains unchanged. This broadening can be explained by the flux-creep model. The flux-creep activation energy at the applied field values has been estimated from the slopes of Arrhenius plots of log rho against 1/T. The flux-creep activation energy has been found to be weakly field-dependent (H¹2/) in high-field regions (>2 T). The upper critical field of the Tl-based superconductor has also been estimated to be 133 T.