The effects of high temperature on the parasitic stage (trophont) and encysted stage (tomont) ofCryptocaryon irritans were examined. The effects on trophonts were examined by rearing fish artificially infected withC. irritansat 25, 28, 31 or 34°C. The effects on tomonts were examined by incubating tomonts at the same temperatures. In these experiments, both the trophonts and tomonts showed normal developments below 31°C, whilst their development was badly damaged at 34°C. The effects of dissolved oxygen on tomont development were examined. Tomonts were incubated in hyperoxic (141%), oxic (100%), hypoxic (24%) and anoxic (0%) conditions. Under the hypoxic and hyperoxic conditions, tomont development was suppressed. Under the anoxic condition, tomonts died. However, when tomonts were transferred into an oxic condition after 2 or 4- week incubation in the hypoxic condition, development resumed, showing a rapid increase in theront excystment rates 10 to 11 days after the transfer. These results show a possibility that the supply of oxygen into water bottom along with the disappearance of thermoclines is involved in the autumn outbreaks of cryptocaryoniasis of cultured marine fish in floating net cages located in inner bays.