Mirnov oscillations and plasma disruptions in the TORTUS tokamak have been studied by using both internal and external magnetic probe arrays. The internal probe was also used to measure the plasma current distribution so that the results could be compared with resistive tearing mode calculations. The growth of m = 3, 4 and 5 modes was found to be consistent with linear tearing mode theory before a disruption but not after it. The observed mode amplitudes, typically bθ/Bθ ∼ 5%, were much larger than theoretical estimates based on the magnetic energy available to drive the modes. Despite the presence of large islands near the limiter, most of the disruptions observed were associated with rapid growth of internal modes.