SUMMARY Halobacterium strains produce a truly extracellular proteinase which degrades gelatine and casein. It has a pH optimum of about 8 and depends upon divalent cations and a high concentration of NaCl or KC1 for activity and stability. Proteolytic enzymes were also found in cell homogenates obtained by ultra sonic treatment. A caseinolytic enzyme, probably different from the extracellular one, is associated with particles which sediment upon ultra- centrifugation. A soluble peptidase of lower molecular weight is also present in the extract. Both enzymes are dependent upon divalent cations and a high con- centration of NaCl or KCI for activity. In contrast to other halophilic en- zymes, the proteolytic enzymes of Halobacterium sdharium are more active in the presence of NaCl than KCl at equimolar concentrations.