Assortative mate choice is, so far, the only demonstrated isolating barrier between colour morphs in fish species flocks, such as freshwater cichlids and marine hamlets, suggesting an important role for sexual selection in speciation. However, there has been little consideration of post-copulatory mechanisms that are known to influence reproductive isolation in other taxa. Selective pressures through a cost to hybrid matings, or genetic drift, may lead to the evolution of fertilization barriers through gamete incompatability. In cases of incipient speciation, such as the hamlets, complete fertilization blocks may not have evolved. Instead, differential fertilization between species may reduce the success of interspecific fertilizations. We examined the fertilization dynamics of sympatric Hypoplectrus nigricans (black hamlet) and Hypoplectrus puella (barred hamlet). Experimental crosses were performed to score fertilization success in within-morph and between-morph crosses. Fertilization success did not differ between self-fertilized, within-morph or between-morph crosses. We therefore found no evidence of post-mating barriers through sperm:egg incompatibilities and potential for self-fertilization. Our results are consistent with a mechanism of colour morph divergence based on sexually selected morph mating preferences.