I investigated male and female sexual strategies of a cardinalfish, Apogon niger, which breeds in sandy areas at sporadic coral colonies. Males mouthbrood an egg mass received from one female at a time. Because of the lengthy mouthbrooding period, the interspawning interval was shorter for females than for males. Females moved between coral colonies to find mates more extensively than did males, and more frequently deserted mates after spawning. The females shortened their interspawning intervals by changing mates, especially in the late breeding season, when their mobility was highest. Their mobility was positively correlated with their disappearance rate, suggesting that mate search increases mortality. This may reduce competition between females for males, resulting in an unbiased rather than female-biased operational sex ratio. Males, on the other hand, practised filial cannibalism of entire broods, which might allow them partially to compensate for the lack of food during the mouthbrooding period. The reproductive loss entailed by filial cannibalism could be effectively offset if males remate soon afterwards. However, cannibal males took a long time to remate because few females were available. Filial cannibalism was less frequent than in a boulder-dwelling congener in which males have easy access to mates. Low mate availability may inhibit male A. niger from committing filial cannibalism. Copyright 1999 The Association for the Study of Animal Behaviour.