Evaporation of an aqueous solution of cytosamine triacetate, C24H36N4O9, yielded prismatic crystals of the orthorhombic space groupP212121, witha= 8.738,b= 12.654 andc= 25.596 Å. The crystal structure was determined by three‐dimensional X‐ray diffraction methods from 2411 independent reflexions measured on an automatic diffractometer. Using the symbolic addition method, the structure was solved from a series ofEmaps and Fourier syntheses, and refined by least‐squares calculations to a finalRvalue of 6.3%. The distances and angles of the cytosine moiety of the molecule do not compare well with those found in cytosine monohydrate and cytidine. The discrepancies are attributed to the influence of hydrogen bonding on the double bond character of the carbonyl bond C*(2)‐O*(2) and on the influence of theN4‐acetylation on the ring bonds. The structure exhibits only one hydrogen bond between the N*(4) and O*(2) atoms of the cytosine moiety (2.864 Å). The molecules pack in antiparallel chains aligned with theaaxis which are held together by van der Waals forces. The basic unit of each chain consists of two molecules related by the screw axis parallel to a and repeated to form an interconnected hydrogen‐bonded herring