We have grown a new alloy semiconductor (Ga1-xErx)As by low-temperature molecular beam epitaxy (LT-MBE) on GaAs (0 0 1) substrates, and have studied its structural, optical, electrical, and magneto-optical properties. By LT-MBE at 200-300 degreesC, homogenous ternary alloy (Ga1-xErx)As thin films with (Erbium) Er concentration from 2 x 10(16) up to 7.7 x 10(20) cm (-3) (x = 3.5%) can be grown without major defects or clustering. (Ga, Er)As is highly resistive, suggesting that EI is 3 + substituting the Ga sublattice. Photoluminescence (PL) at 1.54 mum was observed for the (Ga, Er)As samples grown at 400 degreesC. From magneto-optical spectra, it was found that there is little hybridization of 4f and sp orbitals in (Ga, Er)As, in contrast with the strong sp-ri hybridization in (GaMn)As.