It is shown here that the Rayleigh–Taylor instability in a device like a theta pinch has two different stages. The first stage is during the implosion phase of the theta pinch, and the second stage initiates after the formation of the plasma column. Termination of the first stage and evolution of the second stage from growth to a highly nonlinear stage and eventual wave breaking is presented by holographic interferometry. Though the instability in the first stage has been investigated before, here by employing microscopic theory it is shown that the magnetic field gradient is the driving mechanism of the instability in the latter stage.