The three-dimensional structure of blasticidin S hydrochloride pentahydrate, a member of the cytosine amino nucleoside antibiotics, has been solved using diffractometer data and refined to an R value of 0.115. The crystal data are a = 13.500(5), b = 20.387(7), c = 4.824 A, beta = 98.66(3) degrees, Z = 2, Dc = 1.389 g .cm-3, space group P21. The nucleoside base conformation is anti(chi = 86 degrees) and the 2',3'-unsaturated pyranosyl sugar exhibits a half-chair (degree H5) conformation. The amide plane is twisted from the trans position by about 10 degrees. The guanidium group and the amino group of the amino acid chain are positively charged, while the carboxyl group of the sugar is ionized. The chloride ion is surrounded by water molecules only, in a trigonal prismatic arrangement. The molecule has an extended conformation and there is an intramolecular hydrogen bond between the ammonium group and the carboxyl group. A striking feature of blasticidin is that all the hydrophilic groups lie on one side of the molecule and the hydrophobic groups on the other. Amicetin also shows a similar feature and this might be linked to the commonality of their antibiotic functions. Hydrogen bonds link the hydrophilic sides of adjacent molecules forming double chains parallel to the b-axis. The hydrophobic sides of adjacent double chains are separated by a water layer.