Seasonal trends of adult moth assemblages were investigated using portable light traps in a cool-temperate region in central Hokkaido, northern Japan. Light traps were set at monthly intervals from April to December 2005 in five stands. Seasonal changes in the numbers of species and individuals in each stand were unimodal with a peak in summer (July or August). The value of a similarity index between samples from successive months in each stand was always low, indicating that species composition changed greatly between successive months. Based on the seasonal occurrence of 248 species, the mean occurrence period in each species was only 1.8 months. Among these species, 91.5% were estimated to be univoltine and only 8.5% were estimated to be multivoltine. Most species occurred in the summer (July and/or August), although some occurred only in the spring or autumn. Thus, in the present study the high species turnover of adult moths during the active season was due to the short occurrence period of each species, which may be associated at least in part with univoltinism, synchronized adult eclosion, and short life spans of adult moths.