Detection of the presence of a narrow band, known communication signal of unknown amplitude, phase and Doppler shift in noise is usually based on matched filtering. If, however, the signal propagates through a multipath channel, causing a model miss match, such a procedure exhibit significant performance degradation. In this paper we study signal detection in a Ricean multipath channel. We first identify the conditions under which the conventional, quadrature matched filter detector exhibits significant performance degradation when used in a multipath environment. Then, we develop a generalized likelihood test (GLRT) detector for a narrow band constant modolus signal in a fast fading Ricean channel. We show that the resulting test statistic consists of an appropriate superposition of the quadrature matched filter output and the energy detector statistics. As such, the complexity of the new detector is about the same as the conventional one, while its performance in severe multipath conditions is much better.