It has been known since 1930 that Pieris canidia is distributed in the Tsushima Islands located close to Korea. To consider natural history of this species some surveys were made from the zoogeographical and other viewpoints, and discussions are given as much as possible. 1. The form found in the Tsushima Islands is also common in Korea and the Southern Manchuria, and its range is separated considerably from the neighbouring one which extends from Formosa and Central China to Indo-China (Fig. 1). 2. Comparing with the nominate subspecies distributed from Formosa and C. China to IndoChina, the form from the Tsushima Islands is distinctly smaller and having several different wing characters as follows. On upperside of forewing inner margin of apical patch is more weakly insized at space 5, and marking of space 3 is larger and more conspicuous. Marginal spots on upperside of hindwing are smaller, and ground colour of underside of hind wing white, looks more whitish owing to reduction of black scales (Fig. 3). 3. As for the subspecific name for the Tsushima form, kaolicola Bryk, 1946 (type locality: Genzan, N. Korea) should be used, since there are not conspicuous differences between the Korean form and the former. The name juba Fruhstorfer should not be used any more for the Tsushima form of canidia. 4. In the Tsushima Islands they seem to prefer coastal area to inland area for habitats (Fig. 4). Adults fly at sunny site near the margin of forest generally, and are most abundantly seen at such environment where one side is steep slope covered with forest and the other side is flat openland. 5. As the host plants the following cruciferous species which are mostly prehistoric naturalized or naturalized plants are so far known: - Cardamine flexuosa, Lepidium virginicum, Nasturtium officinale, Rorippa indica and R. dubia. In addition, Arabis stelleri var. japonica and Cardamine scutata which are both wild plants were newly confirmed. The former plant inhabits coastal environment where adults are most frequently seen. As it was ascertained that many larvae were living on this plant, it is presumed to be the basic host plant of canidia (Fig. 5). 6. They seem to have six generations per year. Adults are most abundant in the first generation appearing in middle June (Fig. 6). 7. Oviposition takes place on upperside or underside of leaf. Younger larvae feed on underside of leaf. Older larvae climb up stem and eat fruit from tip, and keep body upside-down when not feeding. 8. It is presumed that canidia reached the Tsushima Islands from Korea along coastal area, depending upon Arabis stelleri var. japonica, in the past geologic age when the sea level was much lower.