Eelgrass populations are currently infected with a disease that produces symptoms and epidemiology reminiscent of the famous eelgrass wasting disease of the 1930s. This disease virtually eliminated eelgrass (Zostera marina L.) in the North Atlantic for three decades. For 50 years scientists have speculated about the cause of the 1930s eelgrass decline. We have now proven that the causal organism of the present epidemic is a pathogenic strain of Labyrinthula, which was suspected, but never conclusively shown to cause the 1930s wasting disease. We have isolated the infectious form of Labyrinthula from eelgrass from Maine to North Carolina on the Atlantic coast, and from Puget Sound on the Pacific coast; disease-related dieoffs of eelgrass beds are confirmed in Maine, New Hampshire, and Massachusetts.