The Seto Inland Sea is the largest enclosed sea in Japan (22,000 km(2) surface area), and is very shallow (average depth of 37 m). Large Zostera marina L. beds throughout the Sea play an important role in its ecosystems and environments. This study reviews the long-term changes in the area of the Zostera beds, as well as some direct and indirect environmental factors which influence their distribution. In 1924, the area of the beds was 4,137 ha in the waters of the Okayama Prefecture, By 1989, 87 % of these beds had been lost. Industrialization and urbanization around the coasts of the Seto Inland Sea began in the 1950s. In the 1960s, the area of the Zostera beds in this Sea amounted to 22,625 ha. Since 1977, 70 % of these beds has been lost. This decline is attributed to: a) reduced water transparency as a result of nutrient load increases and intensive dragnet operations (reversible effects: modification); and b) reclamation by coastal development and port construction activities (irreversible effects: destruction). It is estimated that the former was responsible for the loss of 44 % of the Zostera bed area, while the latter destroyed 40 % of the Zostera bed area in the Okayama Prefecture, between 1924 and 1977. Both types of destructive effects have been restricted since the enactment of a law on the environmental conservation of the Seto Inland Sea in 1973. However, most of the Zostera beds have not recovered. Only in Ajino Bay in the Okayama Prefecture has there been an extension from 192 ha in 1977 to 710 ha in 1991. Limited reclamation activities, increased transparency, and the prevention of drag-net operations by the laying of artificial reefs have made regeneration possible.