In summer, the concentrations of dissolved organic nitrogen compounds are often higher than that of inorganic nitrogen. Under such conditions it would be advantageous if phytoplankton species could utilize organic nitrogen sources, including free or combined amino acids, in addition to inorganic nitrogen. This study focused on histidine, the degradation of which potentially yields three nitrogen atoms for each molecule of histidine. In this work, histidase from Dunaliella tertiolecta, a deaminating enzyme catalysing the first steps of histidine degradation, was purified 4000-fold and partially characterized. The molecular weight of the native enzyme was estimated to be 155 kDa, corresponding to four subunits of 38 kDa. D. tertiolecta histidase is stable in the presence of dithiothreitol and is inactivated by cyanide. Histidinol phosphate, histidine and Mn2+ are effective protectors against cyanide inactivation. The enzyme did not exhibit classical Michaelis-Menten kinetics but showed a relationship between the rate of catalysis (V) and the concentration of substrate (S) that was characteristic of negative allosteric behaviour. A Hill coefficient of 4 was measured for histidine concentrations higher than 22.5 mM. Guanine, xanthine and cytosine nucleotides are inhibitors of D. tertiolecta histidase.