The 1:2 magnesium ion complex of the antibiotic calcimycine (A. 23187) crystallizes in the space group P21, with cell dimensions a = 11.152(2), b = 27.684(2), c = 10.168(2) Å, β = 107.02(2)°, and Z = 2. The structure was refined to an R = 0.063 based on 3473 observed reflections. The octahedral coordination of the Mg2+ ion is highly symmetrical. Each calcimycine ion is bound to Mg2+ through a carboxyl oxygen, a carbonyl oxygen, and the nitrogen of the benzoxazole ring system. The Mg—O distances are respectively 2.00 Å and 2.06 Å, the Mg—N distances 2.23 Å [Formula: see text].