In order to simulate the Snowball Earth conditions that may have existed during the late Proterozoic we have conducted a series of GCM simulations using a simple 50-meter slab ocean, a reduced solar constant of 6% and varied CO2 concentrations. In this study, we vary the CO2 concentration from 100 to 3400-ppmv and use rotation rates corresponding to 18 and 24-hour day-lengths. We also examine the effects of increasing the poleward transport of heat by the oceans. Our results show that below a critical value of approximately 1700 ppmv of atmospheric CO2, sea-ice and sub-freezing temperatures occur from the poles to the Equator. A global mean annual two meter air temperature of 221 ¿K is found for boundary conditions of 100 ppmv atmospheric CO2, 6% reduction in solar forcing and a rotation rate corresponding to an 18 hour day. These results confirm those of earlier studies suggesting or implying that low-latitude glaciation occurred during the late Proterozoic. However, since the ocean is the critical factor for low-latitude glaciation, the results should be view cautiously because of the simple slab-ocean used in this study. ¿ 1999 American Geophysical Union