Crystals of ethidium bromide (2,7-diamino-9-phenyl-10-ethylphenanthridinium bromide) monohydrate are monoclinic,a = 9·577,b = 10·698,c = 20·242 A, β = 106·34 °,Z = 4, space groupP21/c. The structure was determined with CuKα. diffractometer data by direct methods, and was refined by full-matrix least-squares methods toR = 0·058 for 2151 observed reflexions. The phenanthridinium ring is slightly non-planar; the phenyl ring makes an angle of 97 ° with the mean plane of the phenanthridine ring, and the ethyl group is rotated about the N-C bond 84 ° out of the mean plane. One of the amino nitrogen atoms has a pyramidal and the other a planar configuration. Only three of the six active hydrogen atoms participate in a hydrogen-bonding scheme, which involves the pyramidal nitrogen atom, the water molecule and the bromide ion. The phenanthridine rings of two molecules related by a centre of symmetry are separated by 3·50 A.