This paper studies the problem of diversity combining for frequency-hopped multiple access (FHMA) systems that operate in a mobile satellite environment characterized by frequency-nonselective Rician multipath fading. The modulation scheme considered is the incoherent M-ary frequency-shift keying (MFSK). The optimal diversity combining rule is derived under the assumptions that the number of active users (K) in the system is known, all users are chip (hop)-synchronous, and each user employs a random FH address. We suggest practical implementations that are close approximations of the optimal rule and examine the effects of various system parameters on the resulting receivers. The bit error probability performance is analyzed and numerical examples are provided. The effects of the diversity order (L), the signaling size (M) and unequal received powers are examined and related system design concerns such as system capacity and spectral efficiency are evaluated as well.