Nitrate transporters have now been cloned from bacteria, fungi, algae and higher plants, and those identified to date belong to three distinct transporter superfamilies: the ABC (ATP binding cassette) superfamily, the POT (proton dependent oligopeptide transporter) superfamily and the MFS (major facilitator superfamily). The crnA gene from the fungus Aspergillus nidulans and the nar-3/nar-4 genes from the green alga Chlamydomonas reinhardtii encode high affinity NO₃/^- transporters belonging to the MFS, and recently homologues of crnA have also been identified in higher plants. In this paper we review our current state of knowledge regarding the structure, evolutionary relationships and regulation of the crnA family of NO₃/^- transport proteins.