AbstractPredominant self‐pollination was confirmed in mixed populations ofTrillium apetalon(4x = 20, SSUU) andT. smallii(6x = 30, K1K1SSUU). In pollination tests conducted on both species from the Muroran (Mr) population, many good seeds were obtained from bagged flowers as well as open‐pollinated intact flowers. In striking contrast, fruits from castrated open‐pollinated flowers failed to give any good seeds. InT. smalliiin the Usu (Us) population, the same results were obtained for bagged and open‐pollinated intact flowers. On the other hand, fruits from castrated open‐pollinated flowers yielded only a few good seeds. Although both species were experimentally cross‐compatible in both directions, no interspecific hybrids have so far been found in nature. It may be possible that both species are isolated by predominant self‐pollinatioin even though their flowering sea