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The 1.7 Å resolution structure of RNase Sa complexed with the polypeptide inhibitor barstar is reported here. The crystals are in the hexagonal space group P65 with unit-cell dimensions a = b = 56.9, c = 135.8 Å and the asymmetric unit contains one molecule of the complex. RNase Sa is an extracellular microbial ribonuclease produced by Streptomyces aureofaciens. Barstar is the natural inhibitor of barnase, the ribonuclease of Bacillus amyloliquefaciens. It inhibits RNase Sa and barnase in a similar manner by steric blocking of the active site. The structure of RNase Sa is very similar to that observed in crystals of the native enzyme and its complexes with nucleotides. Barstar retains the structure found in its complex with barnase. The accessible surface area of protein buried in the complex is about 300 Å2 smaller and there are fewer hydrogen bonds in the enzyme–inhibitor interface in RNase Sa–barstar than …