Stars with a core mass greater than about 30 M ☉ become dynamically unstable due to electron-positron pair production when their central temperature reaches 1.5 2.0×109 oK. The colapse and subsequent explosion of stars with core masses of 45, 52, and 60 M ☉ is calculated. The range of the final velocity of expansion (3400 8500 km/sec) and of the mass ejected (1 40 M ☉) is comparable to that observed for type II supernovae. A dynamical model of convection is derived and included in the calculations. It was found that the effect of the convection on the explosions is probably not important.