The B-hordeins are the major group of prolamin storage proteins in barley (Hordeum vulgare L.) and they are encoded by a small multigene family that is expressed-specifically in the developing endosperm. We report the complete nucleotide sequence of a clone of one B-hordein gene (pBHR184).The cloned gene contains no introns and belongs to the Bi sub-family of Bhordei n genes. Comparison of the 5'-flanking sequences of pBHR184 with those of related S-rich prolamin genes from wheat shows that several short sequences within 600 bp upstream of the translation initiation codon are strongly conserved. A sequence that is conserved at around -300 bp in the S-rich prolamins is also conserved at similar locations in genes encoding the two major classes of maize prolamin (the Z19 and Z21 zeins) and appears to be unique to prolamin genes. We discuss the possible role of this '-300 element' in the control of gene expression in the developing cereal endosperm.