Ants, as major nectar thieves, often have a negative effect on pollination, and thus plants are expected to possess mechanisms to deter ants from flowers. We found that the corollas of hornet-pollinated Codonopsis lanceolata (Campanulaceae) and the tepals of bee-pollinated Fritillaria koidzumiana (Liliaceae) are slippery to ants; because the flowers of both species have exposed nectaries, slippery perianths may confer defense against nectar-thieving ants. The slippery zones were coated with epicuticular wax crystals, and, when wiped with a solvent, the flowers lost slipperiness. Artificially bridging the slippery zones allowed ants to enter flowers more frequently, and experimental introduction of live ants to the Codonopsis flowers evicted hornet pollinators and shortened the duration of pollinator visits, although no differences were found in the fruit or seed sets of flowers with and without ants. Floral defense based on slippery surfaces may have been overlooked, but could be widespread amongst flowering plants.