A theor. model of the 61.5 K transition in Ni-I boracite is presented. Based on a 6-dimensional irreducible corepresentation of the F̅43cl' paramagnetic group, it shows, within the framework of the Landau theory, that the simultaneous onset of the magnetic ferroelec. and ferroelastic properties can be foreseen from the symmetry of the transition order-parameter. The primary order-parameter is identified as a latent antiferromagnetic ordering. The weak magnetization, polarization and strain components originate in an improper coupling of these quantities with the antiferromagnetic sublattice magnetization. The corresponding magnetic, dielec., elastic and magnetoelec. anomalies are briefly discussed.