ABSTRACT: Wood-boring activities by various invertebrates control the availability of food and space in marine sunken wood communities. We investigated the individual stages of wood fragmentation through a 4 yr colonization experiment. We placed Japanese cedar logs on the sea bed ~2 m below the surface of Tanabe Bay, Japan. A cluster analysis showed 6 successive stages (plus 1 alternative second stage) in the development of the wood borer’s assemblage. Individuals of the bivalve families Teredinidae and Pholadidae and the isopod family Limnoriidae settled on the logs within 2 mo (Stage 1). After rapidly fragmenting the inside of the logs (Stage 2), most of the teredinids died during the first year, leaving numerous empty tunnels reinforced by calcium carbonate linings (Stage 3). Because of this reinforcement, as well as due to the fact that the tunnels never crossed each other, the resulting honeycombed structure remained stable for about 3 yr, allowing for the ongoing development of the sunken wood community. Large-scale fragmentation finally continued with the limnoriids intensively disintegrating the logs from the surface (Stage 4). As the fragmentation process drew to a close, the pholadids disappeared from the assemblage before the limnoriids (Stage 5), the latter persisting until the log had been turned entirely into small particles (Stage 6). This rapid and dynamic fragmentation process is not universal in the sea, but serves as a useful framework for comparing the wood-boring activities across various conditions. Those comparisons will help to evaluate the role of coarse woody debris in marine ecosystems.