Molecular beam epitaxy and optical investigation of Zn 1Ϫx Mn x Se epilayers over a large composition range (xϭ0 -0.78), grown on GaAs ͑001͒ substrates, are reported. The far-infrared ͑FIR͒ reflectance and Raman scattering were performed to characterize the film quality and study their optical and electrical properties. FIR and Raman data provide experimental evidence on the intermediate-mode phonon behavior for Zn 1Ϫx Mn x Se with x up to 0.78. Theoretical modeling fits of FIR spectra lead to the determination of optical parameters such as mode frequency, strength, damping constant, and electrical properties of dielectric constant, carrier concentration, mobility, conductivity, and effect mass, and their dependence on the Mn composition. Values of force constants of F MnSe ϭ17.1ϫ10 4 dyn cm 1 and F ZnSe ϭ14.7ϫ10 4 dyn cm Ϫ1 , and the high-frequency limit dielectric constant of MnSe, ⑀ ϱ ϭ5.4, are obtained. ͓S0163-1829͑99͒07147-7͔