The refractive indices of the ternary

semiconductors CuAlSe
2, AgGaSe
2, CuGaSe
2, and AgInSe
2have been measured over most of the transparency range of these crystals. The optical nonlinear coefficients for second-harmonic generation of AgGaSe
2CuGaSe
2, and AgInSe
2have also been measured. Three-frequency colinear phase matching is analyzed in detail for AgGaSe
2. The birefringences of the other three crystals are not sufficient to permit three-frequency colinear phase matching within the range of the measured index. The merits of AgGaSe
2for nonlinear optical applications are evaluated in comparison with other promising infrared nonlinear materials.