Ritterazines B and C, dimeric steroidal alkaloids related to the cephalostatins, have been isolated from the tunicate Ritterella tokioka and their structures including absolute stereochemistry have been elucidated by spectral and chemical methods. Ritterazines B and C showed potent cytotoxicity against the P388 murine leukemia cells with IC50's of 0.018 and 9.4 ng/mL, respectively.