cDNA probes encoding the barley endosperm ADP-glucose pyrophosphorylase (AGP) small subunit (bepsF2), large subunit (bepl10), and leaf AGP large subunit (blpl) were hybridized with barley genomic DNA blots to determine copy number and polymorphism. Probes showing polymorphism were mapped on a barley RFLP map. Probes that were not polymorphic were assigned to chromosome arms using wheat-barley telosomic addition lines. The data suggested the presence of a single-copy gene corresponding to each of the cDNA probes. In addition to the major bands, several weaker cross-hybridizing bands indicated the presence of other, related sequences. The weaker bands were specific to each probe and were not due to cross-hybridization with the other probes examined here. The endosperm AGP small subunit (bepsF2) majorband locus was associated with chromosome 1P and designated Aga1. The endosperm AGP large subunit (bepl10) major-band locus was mapped to chromosome 5M and designated Aga7. The endosperm AGP large-subunit minor bands were not mapped. The leaf AGP large-subunit major band was associated with chromosome 7M and designated Aga5. One of the leaf AGP large-subunit minor bands was mapped to chromosome 5P and designated Aga6. A clone for the wheat endosperm AGP large-subunit (pAga7) hybridized to the same barley genomic DNA bands as the corresponding barley probe indicating a high degree of identity between the two probes.