Egg production by the freshwater gill monogenean Discocotyle sagittata infecting rainbow trout (Oncorhynchus mykiss) was monitored at 5, 13 and 18°C; eggs were incubated at 6, 10, 13, 15, 16, 18, 22 and 26°C and the hatching times determined. Mean per capita output at 13°C ranged from 0·8 eggs/worm/day (e/w/d) to 18·3 e/w/d for 25 hosts (burdens 1–17 worms/host). Average egg production rates for worms in single burdens varied from 3 e/d to 14·6 e/d at 13°C, mean 7·6 e/d (±0·63) (n=8). Egg production was temperature dependent, increasing from a mean of 1·5 e/w/d at 5°C to 12·0 e/w/d at 18°C. Developmental rate was also temperature dependent: larvae began hatching after 84 days at 6°C, 28 days at 13°C, and 20 days at 18°C. Eggs laid within the same 24 h period hatched over 3–7 days at 13–18°C; this range was extended at decreasing temperatures. Egg viability was generally high: between 13 and 18°C a mean of 92% of larvae successfully emerged. However, viability declined at the highest and lowest temperatures (23% hatching at 6°C, 55% at 8°C and 47% at 26°C). The implications for seasonal parasite transmission are discussed.