Abstract. Trophonts of Cryptocaryon irritans Brown from infected three‐spot damselfish, Dascyllus trimaculatus Ruppell, were kept at temperatures ranging from 7 to 37°C to observe encystment and development of the tomites. At 30, 25 and 20°C, the percentage of trophonts that had encysted in 16 h were 70, 77 and 64% respectively; at 37°C, 44% encysted and at 7°C only 10% had encysted. The optimum temperature for excystment was 30°C; 50% excysted in 5 days and 100% in 7 days. At 25°C, 60% of the tomites started to excyst on the eighth day, and 70% on the ninth day. At 20°C, 10% started to excyst on the ninth day, reaching 40% on the tenth day. No excystment occurred at 37 and 7°C. Newly encysted tomonts were placed in various dilutions of sea water (31 %0) and kept at temperatures ranging from 7 to 37°C. Low salinities, i.e. 16%0 and lower caused tomonts to rupture. At 37, 20 and 7°C, 35% of the tomonts started to rupture immediately in 50% sea water, while at 30 and 25 C, 30% of the tomonts ruptured in 25% seawater. However, none of the cysts developed normally at these dilutions. The percentage rupturing increased with decreasing salinity. Copyright © 1979, Wiley Blackwell. All rights reserved
