Ammonium (NH(4) (+)) transport is a key process in nitrogen metabolism. To elucidate the role of ammonium transporters in the nitrogen consumption of the salt-resistant green alga, Dunaliella viridis, two ammonium transporter genes, DvAMT1;1 and DvAMT1;2, were isolated from cDNA libraries of D. viridis. DvAMT1;1 and DvAMT1;2 share only 40% amino acid identity, indicating that they have highly divergent coding sequences. Functional complementation in a yeast mutant defective in ammonium uptake indicated that both DvAMT1;1 and DvAMT1;2 were functional ammonium transporters. Quantitative RT-PCR showed similar expression patterns, but different transcript abundance levels, for DvAMT1;1 and DvAMT1;2 under different nitrogen conditions. Both were induced at low nitrogen and inhibited at high nitrogen concentrations, especially when NH(4) (+) was the nitrogen source. At the transcriptional level, DvAMT1;1 was diurnally regulated, while DvAMT1;2 was not. In addition, under NaCl concentrations that ranged from 0.5 to 3 M, DvAMT1;1 was down-regulated at the higher salt conditions; conversely, DvAMT1;2 maintained a relatively low, but stable, transcript abundance. The observed differences in transcriptional regulation of DvAMT1;1 and DvAMT1;2 are indicative of their diverse physiological functions in D. viridis.