Résumé: Mate choice is expected to be important for the fitness of both sexes for species in which successful reproduction relies strongly on shared and substantial parental investment by males and females. Reciprocal selection may then favour the evolution of morphological signals providing mutual information on the condition/quality of tentative partners. However, because males and females often have differing physiological constraints, it is unclear which proximate physiological pathways guarantee the honesty of male and female signals in similarly ornamented species. We used the monomorphic king penguin (Aptenodytes patagonicus) as a model to investigate the physiological qualities signalled by colour and morphological ornaments known to be under sexual selection (coloration of the beak spots and size of auricular feather patches). In both sexes of this slow-breeding seabird, we investigated the links between ornaments and multiple indices of individual quality; including body condition, immunity, stress and energy status. In both sexes, individual innate immunity, resting metabolic rate, and the ability to mount a stress response in answer to an acute disturbance (capture) were similarly signalled by various aspects of beak coloration or auricular patch size. However, we also reveal interesting and contrasting relationships between males and females in how ornaments may signal individual quality. Body condition and oxidative stress status were signalled by beak coloration, although in opposite directions for the sexes. Over an exhaustive set of physiological variables, several suggestive patterns indicated the conveyance of honest information about mate quality in this monomorphic species. However, sex-specific patterns suggested that monomorphic ornaments may signal different information concerning body mass and oxidative balance of males and females, at least in king penguins.