A fully differential R-MOSFET-C fourth-order Bessel active low-pass filter employing passive resistors and current-steering MOS transistors as a variable resistor is proposed. The implementation relies on the tunability of the current-steering MOS transistors operating in the triode region which counteract the deviation of resistors in integrated circuit manufacturing technology in order that the group delay of the Bessel active filter can be realized accurately. A 0.75 us group delay 520kHz frequency fourth-order Bessel lowpass filter based on a passive doubly terminated RLC prototype was designed and fabricated using 3.3V power supply and 0.35um CMOS technology. Chip test results demonstrate better than -65dB THD with @ 100kHz, 1.65-Vpp signal, frequency tuning range of more than ten decades from 0.6kHz to 550kHz, chip area of 0.32mm2 and power consumption of 13.3mW.