Properties of midgap levels in n-type GaAs crystals were studied by using trap density spectroscopy (TDS), capacitance-voltage and near infrared (NIR) absorption measurements. The TDS analysis for various n-type samples showed that the concentrations of the midgap level were from 2.0 × 1016 cm−3 to 3.4 × 1016 cm−3. However, those of EL2 in the same crystals were found to be from 1.1 × 1016 cm−3 to 1.3 × 1016 cm−3 by NIR absorption measurements at room temperature. On the other hand, in undoped semi-insulating GaAs crystals with carbon concentrations ranging from 6 × 1014 cm−3 to 2.4 × 1016 cm−3, the densities of the ionized EL2 determined by NIR absorption were only about 30 % of those of carbon acceptor determined by the localized vibrational mode absorption at room temperature. These differences suggest the presence of another midgap donor which does not give NIR absorption. The concentrations of this trap were estimated to be 0.8 to 1.6 times those of EL2 by decomposition of the TDS spectra into their components.