The tunic of Cystodytes dellechiajei (Poly- citoridae), a colony-forming species of the Ascidiacea that contains biologically active alkaloids, was investigated using light microscopy, laser-scanning microscopy and nuclear magnetic resonance techniques. The colonies contain numerous individual zooids, which are embedded in a common tunic. Each zooid is protected by a firm capsule of overlapping calcareous spicules. The colonies lack blood vessels in the tunic, but six morphologically different types of tunic cells were found: pigment cells, bladder cells, vacuolated filopodial cells, granular filopodial cells, morula cells and granular cells. Rod-like bacteria were found in the tunic matrix. Bladder cells and pigment cells could be identified as storage units for acid and pyridoacridine alkaloids, making the tunic inedible and repelling predators. Filopodial cells have long filopodia, which probably are connected to each other. They may be involved in transportation processes within the tunic tissue. The functions of the morula cells and the granular cells are unknown as yet. With its several specialised cells, the tunic of C. dellechiajei represents a dynamic living tissue containing biologically active compounds.