Since 2007, mass mortalities of cultured ascidians Halocynthia roretzi (Drasche) have occurred in Miyagi Prefecture, Japan. The mortalities occur from November through August, and the tunics of affected animals become abnormally weak and soft. The number of farming areas where mass mortalities have occurred has increased rapidly: 3 in 2007, 6 in 2008, and 14 in 2009. When an outbreak of the disease occurred, mortality reached 17 to 100%. Prominent histopathological changes in the diseased ascidians were found in the tunics; the tunics of affected animals were usually much thinner than those of healthy individuals, and the tunic matrix showed marked disintegration with irregular arrangements of fiber layers or the presence of hollow spaces. In addition, flagellate-like cells (10-14 microm x 2-3 microm) stained with hematoxylin were observed in the tunics of 31 out of 36 diseased animals (86%), but not in apparently healthy animals (n=38). Experimental infection with the disease was successfully conducted by immersing small pieces of tunic samples from diseased ascidians into aquaria with healthy ascidians. The flagellate-like cells were confirmed in the tunics of all the experimentally infected animals. These results indicate that the mass mortalities of ascidians accompanied by abnormally softened tunics were caused by an infectious agent, and suggest the involvement of the flagellate-like cells in the disease.