Crystals of Mn3B7O13I boracite with transparent gold electrodes have been prepd. and, after an elec. poling, the single domain state of the sample verified by polarized light microscopy. Ferroelec. hysteresis loops have been measured at room temp. Spontaneous polarization and pyroelec. coeff. of the orthorhombic phase are reported as a function of temp. Piezoelec. measurements have been made using an admittance circle and motional capacitance method. The piezoelec. coeffs. are reported as a function of temp. for the cubic and orthorhombic phases. The effect of the magnetic phase transition on the series resonance frequency is also shown. A measurement has been made on a crystal showing multiple resonances due to the presence of tiny ferroelec./ferroelastic domains to illustrate the necessity of optical control during measurements on ferroelastic compds.