The membrane-bound respiratory complex II, succinate:ubiquinone oxidoreductase (SQR) from Escherichia coli, has been anaerobically expressed, then purified and crystallized. The initial crystals obtained were small and diffracted poorly. In order to facilitate structure determination, rational screening and sample-quality analysis using electron microscopy was implemented. The crystals of SQR from E. coli belong to the trigonal space group R32, with unit-cell parameters a = b = 138.7, c = 521.9 A, and diffract to 2.6 A resolution. The optimization strategy used for obtaining well diffracting SQR crystals is applicable to a wide range of membrane proteins.