A 256-channel electroencephalographic data acquisition system is designed for electrical source imaging of the human brain. The system is microcontroller based, dc-coupled, battery powered, and uses a universal serial bus for communication. An analog to digital converter of 24-bit sigma-delta technology with a sampling rate of 140 Hz and the number of effective bits in conversion is 18, corresponding to a dynamic range of 108 dB. Data is transferred via USB at the rate of 1 Mbyte/sec. The common mode rejection ratio of the system is measured as 102 dB. The performance tests and recorded experimental data show that the developed system can be used in conducting human brain source localization experiments.