A very wide tuning range ultra-low-power MOSFET-C filter is presented. The wide tuning range in this filter has been achieved without using any switchable components or programmable building blocks, and the cutoff frequency of the filter can be adjusted simply through a controlling bias current. The filter has low-pass characteristics with $f_{c}$ = 20Hz to 184kHz while exhibiting a constant power consumption per cutoff frequency over its entire tuning range that is almost four decades wide. The proposed MOSFET-C filter uses PMOS transistors in subthreshold regime for implementing floating and widely adjustable resistors. The ultra high resistivity of the PMOS devices makes them very suitable for implementing very low frequency and compact filters. Realized in 0.18$\mu$m CMOS technology, the filter exhibits a relatively constant noise and linearity performance over its entire tuning range. The active area of the proposed MOSFET-C filter is 0.09mm$^2$.