Two genes, APT1 and APT2, with DNA sequences that exhibit significant sequence identity to yeast and fungal H+/orthophosphate co-transporters, have been isolated from Arabidopsis thaliana. These genes are genetically linked and map to chromosome 5 between markers g4028 and m435. The genes encode almost identical 524 amino acid polypeptides and are predicted to contain 12 membrane-spanning domains. Both APT1 and APT2 are predominantly expressed in A. thaliana root tissues. The level of expression of both genes in roots is regulated by the phosphorus status of the plant, being considerably enhanced when the plants were deprived of an external phosphate supply. The APT1 and APT2 polypeptides are likely to be associated with the membrane transport of phosphate within A. thaliana roots.