The production and the chronobiology of emergence of the cercariae of Euparyphium albuferensis from Gyraulus chinensis experimentally infected with a single miracidium were established during 28 consecutive days from the first day of cercarial shedding. Moreover, the effect of a sudden change in light-dark cycling was investigated. Although the daily cercarial shedding rates show great variability, a progressive increase in cercarial production was observed in the first weeks of the cercarial shedding periods, probably in relation to the demography of intramolluscan larval stages. Under 12:12 light-dark cycling conditions, E. albuferensis cercariae emerged in the light, and the rhythm was circadian. The sudden change in the light-dark cycle resulted in corresponding alterations in emergence patterns, which shows that cercarial emergence was correlated to light-dark alternation. The ecological consequences of daily emergence by mobile cercariae whose target hosts are organisms that regularly occur in the same habitat with the molluscan emitting host, such as those of E. albuferensis, are discussed with reference to the hypotheses proposed to date, and an alternative hypothesis is proposed.