MHD phenomena in ultra-low-q (ULQ; q < 1) discharges are extensively studied by means of three-dimensional magnetohydrodynamic (MHD) simulations. ULQ discharges are operated in the intermediate current range between the tokamak and the reversed field pinch. It is shown that ULQ dynamics is a competitive process between resistive diffusion and MHD activities: the former leads to current profile peaking (dq/dr > 0) and the latter to current profile broadening (dq/dr < 0). The MHD activity of ULQ discharges is studied, and it is found that the key process is non-linear growth of an m = 1 ideal kink instability. It is observed that after the MHD activity has organized a global mode stable state, resistive diffusion dominates the ULQ dynamics on the diffusion time-scale, leading to a global mode unstable state; then, MHD activity appears again.