We conducted transect counts of butterflies in 1999 and 2001 in a coppice on Mt Mikusa, northern Osaka Prefecture, central Japan, to elucidate the effects of experimental mowing on species diversity and assemblage structure of butterflies. At the study site, the forest floor, which had been almost covered with dwarf bamboo in 1999, was mown after the autumn of 2000 to produce alternating 25-m intervals of mown and unmown strips of undergrowth on an experimental basis. Totals of 975 and 775 individuals belonging to 41 and 46 butterfly species were observed respectively before (1999) and after (2001) completion of the strip mowing. Three species of multivoltine satyrids, Lethe diana, L. sicelis and Neope goschkevitschii, dependent on tree sap and dwarf bamboo for their adult and larval food resources, respectively, were dominant in both survey years, although their densities were greatly decreased in 2001. In contrast, annual counts of nectar-dependent and forest herb feeding butterflies, including two species of univoltine fritillaries, Argynnis paphia and Damora sagana, increased in 2001. Thus the stripe mowing of undergrowth enhanced the species diversity of butterflies in the study site coppice by suppressing densities of dominant dwarf bamboo feeders and benefiting forest herb feeders.