An investigation was made of the morphology and life history of an unidentified botryllid ascidian that was first collected from the stony shore of Shikine Island of the Izu Islands in Japan. This species has a very soft and transparent tunic, which is different from that of other botryllids. The arrangement of ovary and testis in this species is the same as those of other species of Botryllus. The embryo develops in the sac-like brooding organ formed in the peribranchial cavity of a zooid. Some colonial parts of significant size that include only the vascular system (i.e., without zooids) often extend from the colony margins, and in these parts vascular budding often occurs. Then, the processes and features of the allorecognition reaction of this ascidian were observed. Allorejection occurred after fusion of the vascular systems between two incompatible colonies. This feature is also observed in Botryllus scalaris, but it takes this ascidian much longer to initiate the allorejection reaction than B. scalaris after fusion of blood vessels between two incompatible colonies. It was concluded, on the basis of this study, that this ascidian should be designated as a new species belonging to the genus Botryllus.