9-Ethyladenine and 1-methyl-5-bromouracil crystallize together in a 1:1 complex. The molecular structure of this complex was solved by analysis of 3-dimensional X-ray diffraction data. The two molecules form a nearly planar hydrogen-bonded pair, connected by two hydrogen bonds, N-H…N from the uracil N 3 to the adenine imidazole N 7 and N-H…O from the adenine amino group nitrogen N 6 to the uracil O 2. The hydrogen-bonded pairs are connected to pairs in neighboring unit cells through additional hydrogen bonds. The crystal structure has an unusual disordering feature. Approximately 6% of the uracil derivatives have an alternative hydrogen bonding to the adenine residue involving the carbonyl oxygen O 4 instead of O 2. The effect of the bromine atom on the hydrogen-bonding interaction as well as its possible mode of action in mutagenesis are discussed.