Summary 1. Winged stages of Ephoron shigae were collected every day during their emergence period during a 6‐year period from 1989 to 1994, by net sweeps or a light trap along the Asahi‐gawa River in western Japan. Emergence occurred mainly in September. 2. Coefficients of determination (r2) were calculated for the regression of the mean date of emergence against cumulative degree‐days during various periods from late March to early September. 3. In both sexes the highest values of r2 were obtained for regression with degree‐days from late June to late August. This indicates that thermal conditions during the late instars affected the emergence timing most strongly. The timing of emergence can be well predicted from the cumulative degree‐days during summer.

1. The winged adults of E/j/iomn^/iigfle swarming overlhe water surface were collected by successive random sweeps during their daily emergence period, and the changes in sex and stage composition were examined, in the lower reach of the Asahi River in western Japan.2. The emergence occurred mainly during a 1 ' /2 h period beginning some time after sunset. Male sub-imagos emerged synchronously and moulted to imagos in a 25 min period immediately prior to an abrupt increase of female emergence.3. This timing of male emergence may be considered the result of selection of males to decrease the risk of death before mating, and for all the males to have an equal opportunity to mate.