The linear and nonlinear optical properties of &agr;‐HIO3have been measured in the visible and near‐infrared. Phase‐matched second harmonic generation is shown to occur for both parallel and orthogonal input polarizations. The observed phase‐matching surfaces are in agreement with theory. In addition, (a) large single crystals of high optical quality are easily grown from water solution; (b) the measured nonlinear coefficientd14is slightly larger thand31in LiNbO3; and (c) no ``optical damage'' effects occur at high power densities. Preliminary results indicate that a substantial number of iodates exhibit similar nonlinear optical properties.