Er-doped GaAs codoped with oxygen was studied by site-selective measurements of photoluminescence spectra and photoluminescence-excitation spectra directly exciting the4f-shell electrons of one kind of Er center at a time. Within one sample, more than 10 kinds of distinctly different Er centers, showing different Er-related spectra, were observed, although its host-excited photoluminescence spectrum is dominated by the luminescence from one kind of Er center. The atomic configuration of that center has been identified as an Er atom located at the Ga sublattice with two adjacent oxygen atoms (Er-2O center). Seven kinds of Er centers in GaAs:Er, O and two kinds of Er centers in AlGaAs:Er, O were studied in detail. The numbers of lines and the magnitudes of energy separations among the lines in each spectrum indicate that the Er atoms of all nine kinds of centers are coupled with oxygen atoms and have noncubic symmetry. The nine Er centers were grouped into several classes according to similarities in their spectra which should reflect symmetries of the Er centers. The atomic configurations of two classes of Er centers are discussed in detail. The Er centers of both classes have a nearest-neighbor site atomic configuration similar to that of the Er-2O center with a symmetry close to rhombicC2v,but Er centers in the two classes have different types of deviation fromC2vdue to the difference in the second-nearest-neighbor atomic configurations of the Er atoms. We also discuss a large difference in the efficiency of the Er-related luminescence under host photoexcitation caused by the difference in the second-nearest-neighbor atomic configurations. ©1997 American Institute of Physics.