The sex ratio of 137 fledglings, from 91 broods, of Blakiston's Fish Owls Ketupa blakistoni raised in Hokkaido, Japan, over the period 1985-99, was analyzed. The sex ratio of fledglings was significantly male-biased (81 males. and 56 females; P=0.04) although no significant difference was detected when it was compared to the expected value (50% males) for each brood size (one or two). Logistic regression analysis did not reveal any significant effects of the locations of breeding sites on the sex ratio of fledglings. Young male owls suffered higher mortality than females. It is suggested that the male-biased sex ratio at fledging is moderated toward parity during the post-fledging stage. Factors considered to attribute to the skewed sex ratio at fledging of Blakiston's Fish Owl were: sexual dimorphism, local resource competition (LRC) and demographic stochasticity resulting from the small population size. Sexual dimorphism suggested that the male-biased sex ratio of fledglings was qualitatively consistent with Fisher's theory. However, sexual dimorphism alone does not quantitatively explain the male-biased sex ratio in Blakiston's Fish Owls. Differential natal philopatry between the sexes suggests that LRC is occurring. The small population size of the Hokkaido population is considered to be the most alarming factor from a conservation point of view.