Cryptocaryon irritans Brown 1951 is a parasitic ciliate of marine teleosts that causes cryptocaryoniasis or 'marine white spot disease'. The life cycle of C. irritans consists of four developmental stages; namely, theront (invasive free-swimming stage), trophont (parasitic stage harboring host epithelial tissues), protomont (stage just after being detached from hosts after full development), and tomont (cystic stage producing and releasing theronts) (Sikama, 1937;Brown, 1963;Colorni, 1985).Cryptocaryon irritans is typically propagated by exposing (or passaging) the parasite on live fish (Colorni, 1985;Burgess and Matthews, 1994;Yoshinaga and Dickerson, 1994;Dan et al., 2006), because in vitro culture methods for their propagation are not available, although the trophonts of this parasite can be grown in vitro, to a certain extent (Yoshinaga et al., 2007). The serial passaging of the parasite on fish is laborious, especially because important events of the parasite (detachment of trophonts from hosts and excystment of theronts from tomonts) occur from late at night to early in the morning (Yoshinaga and Dickerson, 1994;Burgess and Matthews, 1994;How et al., 2015). Furthermore, the passaging on fish must be repeated at almost oneweek intervals, without interruption. This work hinders the progress of studies on C. irritans. To control its daily rhythms can decrease the amount of labor needed for its propagation in laboratories.The daily rhythms of this parasite seems intrinsic, because when light and dark periods were temporarily reversed during the parasitic and cystic periods, the daily rhythms did not show immediate change (Yoshinaga and Dickerson, 1994;Burgess and Matthews, 1994). However, Burgess andMatthews (1994) predicted that the daily rhythms of protomont detachment and theront excystment are influenced by photoperiods. Thus, there is a possibility that the daily rhythms can be controlled by altering photoperiods during the parasitic and cystic periods.In this study, we investigated whether the daily rhythms of the parasite would be controlled by exposing challenged fish and tomonts to different photoperiods.