To enable identification of a time-varying Volterra model using a single input/output realisation, sequences from an orthonormal basis are used to approximate the timevariation. Our work concerns the choice of sequencesfrom a library which comprises several bases. Using a library of wavelet packets allows-the use of the Best Basis algorithm. We apply the Best Basis algorithm in order to select the wavelet packets for approximating the true system's time-variation in the model. The minimum entropy criterion determines the most e@ient basis approximation enabling a minimum number of sequences to be used, leading to a more parsimonious model.