The halophilic archaea display a considerable extent of enzyme diversity. The presence or absence of certain enzymatic activities is closely linked with the taxonomic status of the strains investigated. Thus, Halobacterium species such as Hb. salinarium, Hb. halobium, and Hb. cutirubrum differ from most other Halobacteriaceae tested by the possession of an NAD(+)-dependent glycerol dehydrogenase, by the absence of methylglyoxal synthase activity, and the ability of fermentative growth on arginine. Species such as Hb. saccharovorum and Hb. sodomense, which are still classified within the genus Halobacterium, have an enzymatic machinery greatly different from that of the Hb. salinarium-Hb. halobium group, confirming the need for a taxonomic reappraisal of these species. The presence of NAD(+)-dependent D-lactate dehydrogenase is characteristic of representatives of the genus Haloarcula, which possess only low activities of NAD(+)-independent L- and D-lactate dehydrogenases, if at all. Other enzymes which show considerable diversity are fructose 1,6-bisphosphate aldolase, of which two classes exist, and ribulose 1,6-bisphosphate carboxylase, which is present in a limited number of species.