1. Nitrate reductase has been partially purified from Aspergillus nidulans. It has a sedimentation coefficient of 7.6 (± 0.1) S and a molecular weight of 190000 (± 10000) from sucrose density gradient centrifugation and gel‐filtration studies. There is spectroscopic evidence for the presence of a haemoprotein component.2. It possesses two associated activities, cytochrome c reductase and reduced viologen dye: nitrate reductase. A study of the effects of heat and inhibitors on the three activities suggests that the molecule consists of two functional parts. One of these is heat labile, contains FAD and a haemoprotein and catalyses the transfer of electrons from NADPH to cytochrome c. The other part is heat stable, contains molybdenum and catalyses the transfer of electrons from reduced viologen dyes to nitrate.3. The Km values for nitrate, NADPH and cytochrome c are reported. In each case the Km for one substrate is not affected by varying the concentration of the other, suggesting a lack of interaction between their respective binding sites. Product inhibition by NADP+ and nitrate under saturating and non‐saturating concentrations of NADPH and nitrate suggests a random order rapid‐equilibrium mechanism for the enzyme.