This paper, based on data obtained by field observation, uses the concept of territoriality to explore how the territorial ownership of uninhabited islands in the northern Penghu area has come about, how fishermen protect the resources on uninhabited islands from encroachment, and how the organisation of fishing villages is applied to the management of seaweed gathering on these islands. Among the fishing communities of Penghu, the territorial ownership of uninhabited islands was originally passed on down the generations by word of mouth. It is said that all the uninha­bited islands of northern Penghu once belonged to Ch’ih-k’an (赤崁)village, but the seaweed (Porphyra) growing season coincides with strong northeasterly seasonal winds, and crossing the sea in small wooden boats to gather it has always been fraught with danger. Under these cir­cumstances, it was impossible for Ch'ih-k'an village to retain exclusive rights to all the uninha-bited islands, and inevitably ownership of those farthest from the village was ceded to nearby Hu-hsi (湖西)village and Niao yu (烏嶼)village. But as Hu-hsi village was not close to the sea, possessed few boats and its fishermen lacked experience in navigating the route, the resources of the small islands were encroached upon, giving rise to disputes. As a result these resources were inevitably shared with neighbouring villages, before the territorial ownership of these uninhabited islands was finally clarified. This type of territorial ownership may be described as the result of long-term interaction between the northern Penghu region's natural environment, the techniques of the fishing industry, and the society of fishing communities. At present, some of the uninhabited islands are the sole territory of a single village, while others are the shared territory of several villages. Each village's sole right of utilisation over the islets is limited merely to the winter seaweed growing season. More comprehensive measures to prevent illicit gathering are in place on those islets which produce more abundant seaweed. This reflects the fact that these people's territoriality is time-dependent, and at the same time that the territory can be shared with others. The fishing folk of Penghu regard village temples, inshore fishing grounds and seaweed-producing islets as common property of the community. The management of this common property is operated by village temple organisations made up of village elders. The seaweed­gathering management system includes clear rules as to the qualifications of the gatherers, gather-ing methods and distribution of the seaweed. This management system is regarded as a custom that is taken for granted by every member of society in the fishing communities. It is equitable, is mutually beneficial, shows respect for age seniority, and makes it possible to avoid disputes, as well as having the effect of conserving natural resources.