A fully coupled ocean-atmosphere general circulation model (the Fast Ocean-Atmosphere Model) is used to simulate the Neoproterozoic climate with a reduced solar luminosity (95% of present-day), low atmospheric CO2 (140 ppmv), and an idealized tropical supercontinent. Two coupled simulations were completed with present-day and cold initial ocean temperatures. These experiments are compared with uncoupled (i.e., mixed-layer) model experiments to determine the impact of a dynamical ocean on the Neoproterozoic simulations. In contrast to global sea-ice coverage in the uncoupled experiments, the sea-ice margin seasonally advances to 46 and 55¿ latitude in the coupled experiments. The coupled simulations demonstrate that dynamic ocean processes can prevent a snowball solution and suggest that a reduced solar luminosity and low atmospheric CO2 are not by themselves sufficient conditions for a snowball solution. Heat exchange through vertical mixing in the mid-latitudes, caused by static instability, is identified as the primary process halting the advance of the sea-ice margin. ¿ 2001 American Geophysical Union