We describe results from a 3D adaptive mesh refinement simulation offirst structure formationin a CDM-dominated universe. A spatial dynamic range of >250,000 is achieved, allowing us to see the formation of a collapsing primordial protostellar cloud core set within a cosmological framework. The core mass is∼100 M⊙,comparable to the Jeans mass at the densities and temperatures which prevail in the halo center onceH2cooling has become inefficient. ©1999 American Institute of Physics.