AbstractThe pollination and breeding systems ofCampanula punctataboth in the Izu Islands and the mainland (Honshu) adjacent to the islands were investigated. The predominant pollinators of campanulas are: 1)Bombus diversusin Honshu, 2)Bombus ardensand halictid bees on Oshima Island, and 3) halictid bees on the islands of Niijima, Kodzushma, and Hachijo. The absence of bumble bees exceptB. ardenson Oshima Island, and the abundance of halictid bees represent a kind of island effect. Flowers of the campanula in the Islands were found to be smaller than those of the mainland. This might have resulted from adaptation to the small pollinators. Pollination experiments with plants grown from seeds revealed the following differences in breeding system: 1) most of the campanulas from Honshu were highly self‐incompatible (SI) and a few partially self‐compatible (SC), 2) one half of the plants from Oshima Island were more or less SC and the remaining highly SI, 3) those from Hachijo Island were SC and potentially autogamous. The breakdown of dichogamy co‐operated with self‐compatibility in the development of self‐fertilizing ability in Hachijo plants. Several possible causes for the change in the breeding system including the change in pollinators in the Islands are