A low-voltage fully differential CMOS operational amplifier with constant-gm and rail-to-rail input and output stages is presented. It is the fully differential version of a previously realized single-ended operational amplifier where a novel circuit to ensure constant transconductance has been implemented . The input stage is a rail-to-rail structure formed by two symmetrical OTAs in parallel (the input transistors are operating in weak inversion). The class-AB output stages have also a full voltage swing. A rail-to-rail input common mode feedback structure allows the output voltage control. Measurements in a 0.7 μ standard CMOS process with threshold voltages of about 0.7 V have been done. The minimum experimental supply voltage is about 1.1 V. The circuit provides a 60 dB low frequency voltage gain and about 1.5 MHz unity gain frequency for a total power consumption of about 0.72 mW at a 1.5 V supply voltage.