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AC losses of a monofilamentary MgB2 wire in a self-field are experimentally evaluated at several temperatures and frequencies. The alternating transport currents are applied to the straight sample wire, and the AC losses are observed electrically with a lock-in amplifier. The temperature of the sample wire was adjusted by a conduction cooling with gaseous helium, and the stability was below ??0.1 K near a fixed temperature. The temperature dependence of the transport-current losses was roughly explained with a theoretical expression based on the Bean model, though the experimental results of the transport-current losses slightly depend on the frequency.