In this paper I discuss some long-term continuities in the socio-political dynamics of customary marine tenure in the Melanesian South Pacific. Building on field research material from Solomon Islands, and paying close attention to the pan-Melanesian concept of kastom, I exemplify how customary marine tenure and its social contexts are challenged and transformed by external economic and political pressures. These challenges and transformations are discussed with reference to the emerging legislative contexts of customary tenure rights. General trends are identified for Solomon Islands, particularly regarding the management potential of customary marine tenure. It is argued that the relationship between external challenges andlocal transformations is not one-sided. Certain modern pressures may lead to organizational innovation and reinforce the political base of customary control over marine resources, as expressed by present systems of customary marine tenure.