A design technique is described for continuously time-variable recursive digital band-pass filters for seismic signal processing. Two types of band-pass filters are considered: a cascade of a low-pass, and a high-pass filter, and a direct band-pass filter, with all filters being derived from a continuous unit-bandwidth Butterworth low-pass prototype. Linear interpolation of the filter coefficients between points at which they are known exactly is used to reduce the computational overhead. Data are given for determining the length of the interpolation interval to meet prescribed worst case magnitude and frequency error criteria. A zero-phase response is achieved by filtering in the forward time direction followed by filtering in the reverse time direction. An example is included.--Journal abstract.