This paper presents a new deterministic attack against stream ciphers based on a nonlinear filter key-stream generator. By “deterministic” we mean that it avoids replacing the non-linear Boolean function by a probabilistic channel. The algorithm we present is based on a trellis, and essentially amounts to a Viterbi algorithm with a {0,1}- metric. The trellis is derived from the Boolean function and the received key-stream. The efficiency of the algorithm is comparable to Golic et al.’s recent “generalized inversion attack” but uses an altogether different approach: it brings in a novel cryptanalytic tool by calling upon trellis decoding.