An enzyme catalyzing the deamination of the cytosine moiety of blasticidin S was extracted from a fungal strain that belongs to Aspergillus terreus. The enzyme was purified with ammonium sulfate fractionation, Sephadex G-100 column and DEAE cellulose column chromatography, followed by preparative polyacrylamide gel electrophoresis. Blasticidin S deaminase could be separated easily from co-existing cytidine deaminase by DEAE column chromatography or gel electrophoresis, and preliminary study on the substrate specificity showed that this enzyme acts on blasticidin S derivatives, such as cytomycin and acetylblasticidin S, but not on cytosine, cytidine, purine bases or their nucleosides. Blasticidin S deaminase could be induced by the addition of blasticidin S to the culture, and sulfhydryl compounds, such as mercaptoethanol, were effective in protecting the enzyme from inactivation. The homogeneity of the enzyme was examined by both sedimentation analysis and polyacrylamide gel electrophoresis. The molecular weight and isoelectric point were found to be around 30,000 and 4.35, respectively. Some other properties were also examined.