The recent interest in spatial division multiplexing as a tool to increase optical fiber communication link capacity has recently revived the interest in multi-mode optical fibers. Mode coupling within such fibers is a critical aspect of propagation, for it poses a limit to system capacity. In this paper we analyze fiber twist as a possible cause of mode coupling. It is shown that in a twisted fiber the LP modes undergo a rigid rotation proportional to the twist applied to the fiber. This rotation involves both mode power distribution and mode polarization, generalizing the well known polarization rotation (optical activity) that occurs in twisted single mode fibers. The results of this analysis may enable a more accurate modeling of propagation in randomly coupled multi- and few-mode fibers.