The fruits of the Citrus reticulata family contain high quantities of a fragrant acid, that makes them especially suitable for use cooking usage. In Japan, the most common is the yuzu. Its land of origin is said to be China, but it seems that it was brought to Japan in ancient times (Nara and Heian Period). The yūkō, however, is one of the mandarin varieties that has been rediscovered only recently. It grows in the Doinokubi and Sotome areas of Nagasaki. The presence of yūkō trees has been verified also in other areas near Doinokubi and also in Madarajima, in the Saga Prefecture, in the city of Karatsushi. A member of the Citrus reticulata family, it is considered a new variety of citrus (per Nesumi Hirohisa from the National Agriculture and Food Research Organization (NARO), National Institute of Fruit Tree Science) Because of the name “yūkō” it could be mistaken with a variety derived from the yuzu cultivated in Tokushima Prefecture. But NARO has made it clear that they are morphologically different in terms of external aspects, rind, pulp and seed shape. The mature tree is quite tall. The biggest trees have trunks that are half the height of the tree and its circumference reaches 1.2 m. The branches have short spikes. The flowers are white, with 5 petals. The fruit’s diameter is about 7 cm, with a weight of about 200 gr. When it is ripe both the rind and the pulp become a lemon-like bright yellow (picture). The typical features are its sourness and juiciness. When fully grown, the rind has the sweet perfume of the zabon or the yuzu. It grows spontaneously and does not need agricultural chemicals. Fruits can be freely eaten with the rind and the fact that the whole part of the fruit can be used is the big attraction of the yūkō. Even the traditional usage of the zest comes from the desire to not waste any part of this precious fruit. Also because of the spontaneous nature of the tree, the quality of rind and the size of the fruit differ slightly from one tree to another. The yūkō, in both areas of cultivation, was usually used to prepare marinades or as seasoning for sardines and bluefish, as a substitute to snacks and beverages for children, and also floated in the bathtub, as medicine for colds. Another feature of the yūkō is its long ripening period which starts in September and can go until the following March. From September, when the fruit is green, it can be used as seasoning, and then in November when the fruits begin to change color they can be eaten fresh. In December the ripe fruit’s color turns to yellow. Unfortunately, since the `60s, the yūkō has rapidly been disappearing from the local landscape due several causes: the introduction of Wenzhou mandarin cultivations and the subsequent risk of contamination of the local varieties, the ready availability of easy-to-use of vinegar dressings, the need for space to expand vegetable fields, and the growth of the trees that became so tall making it difficult to pick the fruit. Lastly, the urbanization and transformation of Doinokubi into residential areas has exacerbated this downward trend. Now, first of all, it is necessary to rebuild consciousness about yūkō in its two areas of cultivation. Then production needs to be improved by planting new trees and treasuring the existing ones. Furthermore, recipes and processing methods that make the most of yūkō’s characteristics will be developed as well as improved distribution channels. Other activities may include promotion of green tourism and consolidating links with food education and welfare activities. The historic areas where the yūkō was cultivated, Doinokubi and Sotome, were originally areas where the population lived on agriculture and fishing. the landscapes here are somewhat steep, limiting the cultivated areas, which are mostly planted with sweet potatoes, wheat, millet. Doinokubi is an area 10 km southwest from Nagasaki city center. The Sotome area, sitated to the northwest of Nagasaki, 50 minutes by train from city center (about 40 km), is a place blessed by a rich and beautiful nature. Historically it was the area where many of the so-called “Hidden Christians” lived since Meiji period. Because of the activity of French Father De Rotz, Christian History has become a cultural feature of Sotome area. The two sites have in common the fact that during the Edo period they both belonged to Saga but, being 20 km from one another, it is said that they developed autonomously. It is still not known which of the two areas is the original territory of yūkō. In Doinokubi, it grew in private gardens alongside the pomelo and the summer mandarin and was kept for private usage. However, because it was also found by the roadside it is thought that the seeds were transported by birds. In Sotome, the fruit was also kept for private use. There are those that believe, in this area densely populated by Christians, an inheritance of the Hiding Christians and their religion, that the yūkō was spread by Father De Rotz (1840-1914) who could no longer bear the poverty of the local people.