The Harlequin fish Othos dentex is a demersal teleost that is endemic to temperate Australian coastal waters, in which it is popular with recreational fishers and a valued incidental catch of commercial fishers. A number of demersal carnivores, which co-occur with O. dentex on the south coast of Western Australia, are long lived, reaching ages of over 60 years and are thus particularly susceptible to overfishing. Othos dentex were collected from the waters of this coast mainly by spear fishing. The 380 individuals thus caught were used to determine the length and age compositions, growth, reproductive biology and dietary composition of this species. The maximum recorded age was 37 years, which makes this species potentially vulnerable to the effects of exploitation. A lack of bimodality in sex based age-frequency distributions and the absence of testicular tissue within ovaries and of ovarian tissue in testes demonstrated that O.dentex is a gonochorist, only once previously recorded for an athiine serranid. This is not consistent with some contemporary theories of the evolution of gonochorism from protogony in the Serranidae. Analysis of stomach contents demonstrated that O. dentex was exclusively piscivorous. Stomachs either typically contained an entire, large fish prey, which filled the stomach, or more frequently were empty. The mouth gape of O. dentex was particularly large. The change in the dietary composition of O. dentex with length was related to the body size of the prey rather than to the family of those prey. The above data, together with underwater observations imply that O. dentex is a particularly efficient ambush predator that specifically targets large prey and thus feeds relatively infrequently, optimizing its foraging activities and dietary intake.