Cloeon dipterum (L.) (Ephemeroptera: Baetidae), the common wetland mayfly, emerges and oviposits every season, except winter, and has overlapping generations in the temperate region. We investigated the life history of C. dipterum associated with drought. Field experiments and sampling were conducted in a wetland (25 by 80 m) located in central Korea. Larvae were sampled weekly within two habitat types (a large, deep wetland and a small, shallow wetland) using a dredge sampler, and adults were sampled every 2 d with cube emergence traps, from July 2011 to September 2012 (except winter). C. dipterum had an extended emergence period from late April to early October with four peaks: early May, mid-June, July, and August/September. When the water depth was shallow because of drought, emergence was suppressed and delayed. The accumulated degree-days and body length of the overwintering cohort that emerged in spring 2012 were markedly larger and longer than those of other cohorts. Adults emerged intensively around sunset. C. dipterum has a multivoltine life cycle with four cohorts per year; its life history and population density can be changed by natural disturbances such as drought and fish predation.