ZnSe/ZnMgS distributed Bragg reflectors (DBRs) with a high refractive-index contrast were grown on GaAs (1 0 0) substrates by metalorganic vapor-phase epitaxy. The difference of the refractive indices between ZnSe and Zn0.27Mg0.73S was estimated to be about 0.52 at 510 nm, which is very large compared with previous II-VI DBRs. The maximum reflectivity of the grown ZnSe/Zn0.27Mg0.73S DBRs (with only 5-periods) was measured to be 93 % at 510 nm at room temperature. DBRs with a high refractive-index contrast can reduce the penetration depth of light into the DBR and have the potential to increase the strength of the exciton-photon coupling in a microcavity.