Multiplicative noise is known to be useful in modeling multipath propagation which is important in mobile communication systems. In this paper, nonparametric detection of known signals in multiplicative noise is considered. The locally optimum detector is derived based on si,sns and ranks of observations for good weak-signal detection performance under any specified noise probability density function. This detector has similarities to the locally optimum detector for known signals in multiplicative noise. The asymptotic performance of this nonparametric detector is as good as that of the locally optimum detector, With approximate score functions, we can construct the approximate locally optimum rank detector. (C) 1997 Published by Elsevier Science B.V.