A satellite DNA can be isolated from wheat and barley using Ag/Cs,SO4 gradients. These DNAs are highly repeated, each with a complexity of about 10 bp. The satellites isolated from the two species cannot be differentiated by physical properties such as buoyant density, melting temperature or renaturation kinetics and heterologous hybrids melt at the same temperature as homologous hybrids. The restriction endonuclease MboII digests both satellites to give identical patterns. These data together with those from digests of RNAs complementary to the separated DNA strands suggest a general formulation of a sequence as (GAA)m (GAG),,. Localisation of the satellite by in situ hybridisation shows it to have major sites on all seven chromosomes of the B genome and chromosomes 4A and 7A in hexaploid wheat, and on all barley chromosomes. There are specific minor sites on other chromosomes of the A and D genomes of wheat.