Chemical investigation of the cytotoxic CH2Cl2-soluble extract of the marine tunicate Pseudodistoma arborescens led to the isolation of four brominated indole alkaloids, arborescidines A [1], B [2], C [3], and D [4], which were characterized by their spectral data, especially 2D nmr. Only arborescidine D [4] showed moderate activity (IC50 3 micrograms/ml) in vitro against the growth of KB human buccal carbinoma cells.