The extreme halophilic bacterium Halobacterium cutirubrum was examined for base, nucleoside, and nucleotide aminohydrolase activity on pyrimidine bases and their nucleosides and nucleotides. Only adenosine aminohydrolase activity was demonstrated. The enzyme was extracted from the cells and purified about 70-fold by liquid polymer two-phase fractionation, ammonium sulfate precipitation, and gel filtration. The enzyme has an apparent Km for adenosine of 4 × 10−5 M. The optimum pH is 7.5. The enzyme requires 4.0 M NaCl for maximal activity, but is irreversibly inactivated regardless of the concentration of NaCl when stored for extended periods of time.