In a colonial ascidian Clavelina miniata, physical stimulations induce strong luminescence in the tunic. We described here the tunic cell morphology and bacterial distribution in the tunic that is a luminous tissue of this species. Three types of tunic cells are morphologically discriminated as morula-like tunic cells, tunic phagocytes, and tunic granulocytes, and they correspond, respectively, to the Type I, Type II, and Type III cells described by Aoki et al. (1989). Morula-like tunic cells are similar in morphology to morula cells that are hemocytes commonly found in ascidians. Tunic phagocytes contain round granules, clear vacuoles, and occasionally phagosomes. Tunic granulocytes characterized by a number of elliptical granules and they occasionally contain phagosomes and round granules that are similar in structure to tunic phagocytes. According to the description by Aoki et al. (1989), tunic phagocytes are supposed to be luminous cells. Elongated bacteria of unique forms are found in tunic phagocytes. However, these bacteria are probably not luminous ones, since they also are distributed in tunic granulocytes and outside of the tunic cells. Because other bacteria-like inclusions are not present in tunic phagocytes, we found no structural evidence to support the bacterial origin of bioluminescence in C. miniata. The clear vacuoles of tunic phagocytes may be a possible candidate for the subcellular site producing bioluminescence.