The structure of ethylenedibiguanide Mn(III) nitrate monohydrate has been determined using Mo Kα radiation. The complex is triclinic:P-1, a=9.412(2), b=11.452(3), c=8.635(1) Å, α= 101.10(2)°, β=102.45(2)° and γ=102.13(2)°. The structure was solved by the heavy-atom method, and refined by full-matrix least-squares to an R value of 0.024 for 3369 observed reflections. The disposition and nature of the metal-ligand bond in this and related compounds have been discussed. The crystal structure of the present compound is not in agreement with that proposed from chemical evidence. © 1990 Plenum Publishing Corporation.