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Bmal1 is the key component of the central molecular clock that mediates the circadian control of various biological activities including immune reactions. In fish, the role of Bmal1 in immunity is unclear. In this study, we examined the expressional regulation and potential role of Japanese flounder Bmal1 (named PoBmal1) in pathogen associated immune response. PoBmal1 shares 85% identity with its human counterpart and possesses the conserved helix-loop-helix domain and PAS domain. PoBmal1 transcript was distributed extensively in nine tissues and altered to significant extents during the infection of two bacterial pathogens. PoBmal1 , as well as IL-1β, IL-6, IL-8, and TNF-α, exhibited oscillating expression through the light-dark cycle in a tissue-dependent manner, and the expression patterns of the cytokines were largely similar to that of PoBmal1 , both being higher during the photophase. Knockdown of PoBmal1 reduced the expressions of two clock genes (Cry1 and Cry2) and IL-1β, IL-6, IL-8, and TNF-α in a time-dependent manner. During bacterial infection, fish with PoBmal1 knockdown exhibited significantly increased expression of inflammatory cytokines in a tissue- and time-dependent fashion, which largely correlated with significantly decreased bacterial dissemination in tissues. Taken together, our study revealed that flounder Bmal1 exerted a regulatory effect on inflammation response in a manner that was situation-dependent, and that Bmal1 had a significant impact on the outcome of bacterial infection. These results provided new insights into the immune function of fish Bmal1. • PoBmal1 expression occurred in 9 tissues and was regulated by bacterial infection. • PoBmal1 and inflammatory cytokine expressions oscillated similarly with light input. • PoBmal1 knockdown impaired the expressions of Cry and inflammatory cytokines. • PoBmal1 knockdown enhanced inflammatory cytokine expression during bacterial infection. • PoBmal1 knockdown reduced bacterial dissemination in fish tissues. [ABSTRACT FROM AUTHOR]