The present study is the first attempt at the theoretical calculation of the tautomeric/conformational equilibrium for a zwitterionic alpha-amino acid in aqueous solution. The gas-phase zwitterionic structure for aspartic acid (Asp) does not correspond to a local minimum on the potential energy surface. The optimized geometry was determined for the dihydrated zwitterion at the HF/6-31G* level. Single-point calculations for the isolated zwitterion, with the geometry optimized for the dihydrate, were performed up to the QCISD(T)/6-31G* and MP2/6-311++G** levels. Vibrational-frequency-dependent thermal corrections were calculated for the dihydrates. Relative hydration free energies of the isomeric species were calculated using the free energy perturbation method implemented in Monte Carlo simulations. In a second approach, relative free energies were calculated using the self-consistent isodensity polarizable continuum model (SCIPCM). In aqueous solution, aspartic acid forms two tautomeric zwitterions (alpha and beta), with at least three (two N-C-C-C gauche and one trans) stable rotamers for each. Calculated by the ab initio/Monte Carlo method, the relative value of log K for the deprotonation equilibria of the alpha and beta zwitterions is 1.07 +/- 0.36, as compared to the experimental value of 1.26 at T = 298 K and p = 1 atm. The calculations predicted (with one exception) the rotamer preference of the six studied species in accord with a previous experiment. Relative in-solution free energies obtained with the SCIPCM and ab initio/Monte Carlo methods are similar at the Hartree-Fock level for the lowest-free-energy alpha and beta conformers. Furthermore, the SCIPCM method predicts the free energy separation of the beta-gauche conformers in basic agreement with the experiment. Results suggest that consideration of the correlation energy is more important for comparisons of different iautomers of the Asp zwitterion than for considerations of conformers of a given tautomer.