Two types of sexual cannibalism, differing in the sex of the victim, were found among heterosexual pairs of the parasitic isopod Ichthyoxenus fushanensis Tsai and Dai, 1999, residing in the body cavity of the freshwater fish Varicorhinus bacbatulus (Pellegrin, 1908). In one type, categorized as sexual cannibalism, the male was consumed by the female before or after mating. In the other, reversed type, the female was eaten by her mate during or after breeding. Both types of cannibalism occurred during the breeding season from April to November, with female-on-male cannibalism occurring earlier on, and the reversed type later. Both types of cannibalism occurred in pairs inhabiting smaller hosts (<10 cm in body length), which suggests that resource limitation is an important factor triggering the cannibalism. Cannibalism may lead to rapid growth or sex change of the cannibal. The availability of free-living mancas during the breeding season enables a mating pair to be re-formed after cannibalism has occurred. Both types of cannibalism occurred in mating pairs with a low size ratio (below 1.6, female to male), and both resulted in a greater discrepancy between female and male sizes in the re-formed pairs. This increase in size ratio between paired individuals may ultimately lead to an increase in clutch size. Because an individual of I. fushanensis undergoes protandrous sex change, the cannibalistic behavior could not have evolved in response to selection on either the male or female sexuality. Rather, both types of cannibalism may be regarded as the result of competition between paired individuals, which appears to be a by-product in the evolution of a reproductive strategy rather than a consequence of sexual selection.