Abstract : As the number of breeding pairs depends on the adult sex ratio in a monogamousspecies with biparental care, investigating sex-ratio variability in natural populationsis essential to understand population dynamics. Using 10 years of data (2000–2009) in a seasonally monogamous seabird, the king penguin (Aptenodytespatagonicus), we investigated the annual sex ratio at fledging, and the potentialenvironmental causes for its variation. Over more than 4000 birds, the annual sexratio at fledging was highly variable (ranging from 44.4% to 58.3% of males), and onaverage slightly biased towards males (51.6%). Yearly variation in sex-ratio biaswas neither related to density within the colony, nor to global or local oceanographicconditions known to affect both the productivity and accessibility of penguinforaging areas. However, rising sea surface temperature coincided with an increasein fledging sex-ratio variability. Fledging sex ratio was also correlated withdifference in body condition between male and female fledglings. When moremales were produced in a given year, their body condition was higher (andreciprocally), suggesting that parents might adopt a sex-biased allocation strategydepending on yearly environmental conditions and/or that the effect ofenvironmental parameters on chick condition and survival may be sex-dependent.The initial bias in sex ratio observed at the juvenile stage tended to return to 1:1equilibrium upon first breeding attempts, as would be expected from Fisher’sclassic theory of offspring sex-ratio variation.