The crystal structures of the dipeptides L-alanyl-L-aspartic acid, C7H12N2O5, and alpha-L-glutamyl-L-aspartic acid, C9H14N2O7, have been determined from three-dimensional X-ray diffractometer data. Alanylaspartic acid crystallizes in the orthorhombic space group P2(1)2(1)2(1) with four formula units in a cell of dimensions a = 13.389(5), b = 14.467(3), c = 4.781(1) A. Glutamylaspartic acid also crystallizes in space group P2(1)2(1)2(1) with four formula units in a cell of dimensions a = 13.709(5), b = 16.126(7), c = 4.939(5) A. Both structures were solved by direct methods and refined by full-matrix least squares methods; the final value of the weighted R-factors (on F) were 0.040 based on 790 independent intensities with I greater than or equal to 2 sigma (I) for Ala-Asp and 0.033 based on 1105 intensities with I greater than or equal to 2 sigma (I) for Glu-Asp. Each dipeptide occurs as a zwitterion with the amino terminus protonated and the main chain carboxyl group deprotonated. The conformation of the peptide linkage is trans in both molecules, the omega torsion angle being - 175.9 degrees in Ala-Asp and 174.3 degrees in Glu-Asp. There is considerable intermolecular, but not intramolecular, hydrogen bonding in the crystals. The conformations and structures of Ala-Asp and Glu-Asp are compared to those of other structurally characterized acidic dipeptides.