The use of transistors for the amplification of fractional microvolt signals at extremely low frequencies is illustrated in the design of an amplifier developed for use in a lunar seismograph. The amplifier has an equivalent input noise voltage of 0.2 microvolts, p-p, with a source impedance of 2000 ohms and a 3 db bandwidth of 0.035 cps to 22 cps. The nominal input impedance is 1700 ohms. It is completely transistorized and performs satisfactorily over a specified temperature range of −20°C to +100°C. Low power requirements, high reliability, and capability for remote calibration and gain change make this amplifier especially suitable for any field or remote operation under extreme environmental conditions. This amplifier is presently being used in ocean bottom seismographs and magnetic variometers.