Colony-stimulating factor 1 receptor(CSF-1R) is an important regulator of monocytes/macrophages(MO/MΦ). Although several CSF-1R genes have been identified in teleosts, the precise role of CSF-1R in ayu(Plecoglossus altivelis) remains unclear. In this study, we characterized the CSF-1R homologue from P. altivelis, and named it Pa CSF-1R. Multiple sequence alignment and phylogenetic tree analysis showed that Pa CSF-1R was most closely related to that of Japanese ricefish(Oryzias latipes). Tissue distribution and expression analysis showed that the Pa CSF-1R transcript was mainly expressed in the head kidney-derived MO/MΦ, spleen, and head kidney, and its expression was significantly altered in various tissues upon Vibrio anguillarum infection. After Pa CSF-1R neutralization for 48 h, the phagocytic activity of MO/MΦ was significantly decreased, suggesting that Pa CSF-1R plays a role in regulating the phagocytic function of ayu MO/MΦ. Although several CSF-1 R genes have been identified in teleosts, the precise role of CSF-1 R in ayu (Plecoglossus altivelis) remains unclear. In this study, we characterized the CSF-1R homologue from P. altivelis, and named it Pa CSF-1R. Multiple sequence alignment and phylogenetic tree analysis showed that Pa CSF-1R was most closely related to that of Japanese ricefish ( Oryzias latipes). Tissue distribution and expression analysis showed that Pa CSF-1R transcript was mainly expressed in the head kidney-derived MO / MΦ, spleen, and head kidney, and its expression was significantly altered in various tissues upon Vibrio anguillarum infection. After Pa CSF-1 R neutralization for 48 h, the phagocytic activity of MO / MΦ was significantly decreased, suggesting that Pa CSF-1R plays a role in regulating the phagocytic function of ayu MO / MΦ.