Economou and Kanta (2008) [4] considered equilibrium customer strategies in the observable M/M/1 queue with an unreliable server. It is assumed that the server alternates between on and off periods. Upon arrival, the customers can observe the queue length and will decide whether to join or balk the system according to the information about the server's state based on a linear reward-cost structure. In this paper, the corresponding unobservable cases in which the queue length is unobservable to arriving customers are investigated. The mixed equilibrium balking strategies are derived in two cases, according to the information on the server's state. Numerical examples are presented to illustrate the effect of the information levels on the behavior of the customers. (C) 2014 Elsevier Inc. All rights reserved.