Two high affinity nitrite transporters have been identified in Chlamydomonas reinhardtii. They have been named system III and system IV and shown to be differentially regulated by nitrogen and carbon supply. System III was induced under high CO2 and required a micromolar nitrate signal for optimal expression, was inhibited by ammonium, and was not affected by either chloride or the chloride channel inhibitor 5-nitro-2-(3- phenylpropylamino)benzoic acid. System IV was induced optimally under limiting CO2 and did not require nitrate signal, was inhibited by chloride and 5-nitro-2(3-phenylpropylamino)benzoic acid, but was not affected by ammonium. Two transcripts that shared the expression pattern of systems III and IV activities were detected with an Nrt2;3 gene probe. In addition, a mutant defective in both the activity of system III and the expression of Nrt2;3 gene has been isolated. Genetic crosses and in vivo complementation studies indicate that this mutant is defective in a locus that is closely linked to the regulatory gene Nit2.