NADP-specific malic enzyme (EC 1.1.1.40) has been purified about 160-fold from the moderate halophileVibrio costicola. The enzyme has a molecular weight of about 120 000. The purified enzyme was unstable in dilute solutions but could be stabilised by NaCl or glycerol. NH4Cl or KCl caused maximal activation at 0.1 M, but higher concentrations were inhibitory. NaCl did not activate and was instead a mixed-type inhibitor towards NH4Cl or KCl. The salt concentration affected the kinetic parameters of the reaction. The apparentKmforL-malate reached a minimal value at about 0.1 Msalt; the value for NADP, on the other hand, increased continuously with the salt concentration. The reaction also required a divalent cation activator, Mn2+being better than Co2+or Mg2+. NADH was a mixed-type inhibitor towards both substrates, whereas oxaloacetate was strictly competitive towards L-malate and non-competitive towards NADP. The inhibition kinetics were sigmoidal for NADH and hyperbolic for oxaloacetate. The malic enzyme fromV.costicolawas similar to those of a marinePseudomonasandHalobacterium cutirubrumin kinetic and regulatory properties but showed a response to salts intermediate between those of the latter enzymes.