ABSTRACT. – In this paper, I reviewed current knowledge on the major histocompatibility complex (MHC) in fish, specifically focusing on the MHC IIB class in cyprinid species. The general hypotheses on the origin of MHC genes and the mechanisms maintaining the polymorphism of these genes are presented and exemplified by studies performed on fish. Among them, parasite-mediated selection (balancing selection) and reproductive mechanisms based on mating preferences are the most often cited mechanisms maintaining the extensive MHC polymorphism in fish. MHC polymorphism in fish populations is driven by parasites through overdominant selection or frequency-dependent selection. However, the intermediate number of MHC alleles was documented in individuals with the lowest parasite load. A trade-off between investing in MHC and other components of life maintenance has been hypothesized and documented by inter-species (comparative analyses) and within-species studies. Sexual selection acts in the evolution of MHC polymorphism, where good or compatible genes in the male are generally selected by the female through mating preference. A mating preference for MHC genes has been documented in several fish species and it seems that the MHC diversity of the potential mating partner is evaluated by the olfactory system