Cytidylic acid "a" (cytidine 2'-monophosphate) crystallizes in the triclinic space group P1 with two molecules of cytidine monophosphate (C(9)H(14)O(8)N(3)P) and six molecules of water in the unit cell. X-ray analysis of this crystal shows that in both molecules, the base is in the anti conformation, the ribose ring is C(2')-endo puckered, and the hydroxyl O(5') is gauche-gauche. The two molecules are linked by a short hydrogen bond through the phosphate oxygens.