Xenogeneic rejection reactions were histologically examined among four compound ascidians of the genus Botrylloides; B. simodensis, B. lentus, B. fuscus and B. violaceus, to compare with the allogeneic rejections of these species. When the incompatible conspecifics were brought into contact, hemolytic rejections occurred at the point where the tunic of the two colonies was partially fused. Xenogeneic contact at their growing edges induced hemolytic rejection in some combinations (B. simodensisB. lentus, B. lentus-B. fuscus, and B. fuscus-B. violaceus), while conspicuous reaction was not found in the other combinations. Since the hemolytic rejection requires the partial fusion of tunic, the occurrence of hemolytic rejection suggests that the tunic cuticle of the colonies does not discriminate the facing colony from conspecifics. On the other hand, whereas cut surface contact between incompatible conspecifics induced intense rejection in B. simodensis, it resulted in fusion (formation of vascular connection) even in the combination in which the growing edge contact resulted in rejection. In xenogeneic combination, the cut surface contact of colonies always resulted in an intense rejection reaction except for B. fuscus-B. violaceus in which hemolytic reactions did not occur. The absence of hemolytic rejection suggests that the effector system for rejection reaction is not activated in this combination. Activity of phenoloxidase, a key enzyme of the rejection reaction, indicates lower reactivity in B. lentus, B. fuscus and B. violaceus than that in B. simodensis.