Abstract This paper has dascribed an extremely stable low frequency bandpass filter which is very useful in electroencephalographic signal analysis. The use of filters in EEG signal analysis has been discussed in the literature so only bibliographic references have been mentioned in this paper. The frequency and selectivity dapend only on a few capacitors and resistors because of the use of high gain operational amplifiers. Thus, the stability and calibration dapend only on a few passive circuit elements which can be precision components, rather than on the characteristics of the tubes or transistors. For values of Q in the ordar of 100 or less, the dasign is rather straight forward. The filter characteristics are the same as those of a parallel resonant circuit with the same Q as that of the filter. In the dasign, the bandwidth is constant and the Q of the filter changes as the frequency is adjusted. Filters of this dasign have been satisfactorily used in EEG signal analysis at The University of Texas for about 2 years.