As described in the preceding paper, bostrycin, an antibiotic produced by Boetryconema alpsxtre, was isolated recently with chloroform extraction from the cultured broth of this parasitical mould on PoZygonum fitiforme Thunb. var. neofitiforme Ohwi. This antibiotic is active mainly against some species of Gram-positive bacteria. In order to find out the molecular structure of bostrycin and establish its stereochemistry including the absolute configuration, we have studied the crystal structure of one of p-bromobenzoate derivatives, (C23H190,Br), hereafter XIa, by the X-ray method, parallel with an chemical investigation('). This paper will describe the outline of the structure determination of XIa. Large single crystals of XIa were grown from dimethyl formamide solutions. Crystals, suitable for the structure determination, were ground to a cylindrical form with the solvent. The crystal is found to be monoclinic with the space group P21 and the unit-cell dimensions, a = 13.294 * 02003, b = 26.495 f 0.093, c = 7.225 ? 0.002 i and 6 = 90.63 f 0.01'. In order to explain the chemical composition of the crystal, Cz6Hz601NBr, given by an elementary analysis and the observed density, 1.470 g-cm-', the unit cell should contain in addition to four