Measurements with a magnetic probe, internal coils, soft X-rays and visible light on a small tokamak reveal the presence of tearing modes with m = 4, 3,2, and n = 1. The modes grow, rotate and saturate at a level br/Bθ ⪅ 3%, which gives rise to magnetic islands of width ⪅ 1.5 cm for m = 3. The islands are more elongated at this level than a simple calculation suggests, and field-line calculations and experiments suggest that the surfaces may be broken between them. The island width is predicted by a cylindrical initial-value calculation for the instabilities, using the measured current distribution, saturation level and magnetic Reynolds number. If the m = 3 mode exceeds the 3% level a minor disruption results. An m = 2 mode at a level of 6% gives rise to a major disruption whether or not an internal m = 1 mode is present. No other mode appears to play a significant role in the disruptive process.