While at present the number of stars formed strongly decreases as a function of their mass, recent theoretical investigations suggest that the first generation of stars that formed on the primordial composition, as produced by the Big Bang, might have been dominated by very massive stars of more than 100 solar masses. In contrast to stars that form at present, the first generation of very massive stars may reach the late evolutionary stages without major mass loss: After central helium depletion, these stars encounter the electron-positron pair creation instability which leads to explosive burning of oxygen and, later, silicon. For a 250 solar masses star, e.g., we find an explosion energy of about 70 times that of typical core collapse supernovae and more than 40 solar masses nickel are expelled which radioactively decays into iron. For more massive stars photo-disintegration lead to collapse into a black hole instead of disruption. We present the results of a two-dimensional hydrodynamic calculations of the collapse of a rotating 300 solar mass star and possible observational consequences.