Production of the cercariae of Centrocestus formosanus and Haplorchis pumilio was not affected by a 10-day starvation of the snail host Melanoides tuberculata, and there was no circadian rhythm in the pattern of cercarial emergence. Daily average cercarial productions from each snail were 1,643 for C. formosanus and 689 for H. pumilio at 25 C and 500 lux illumination. The number of cercariae was moderately correlated with the size of snail host. The heaviest sheddings ever observed from 1 snail in 24 hr were 63,400 cercariae for C. formosanus and 3,470 cercariae for H. pumilio. The cercariae of C. formosanus could emerge in the dark, but the number was much less than in the light. In contrast, the cercariae of H. pumilio emerged equally well with or without light. Within the temperature range tested, the emergence of cercariae occurred at 15-35 C, but not at 10 C. Shedding of C. formosanus cercariae increased with the rise in ambient temperature, whereas the greatest shedding of H. pumilio cercariae occurred at 25 C. Life-span of the cercariae was temperature-dependent. The most favorable temperature for survival of C. formosanus was 15 C, at which some survived for 160 hr, and 20 C for H. pumilio, with the longest survival of 130 hr.