six new cytotoxic tetracyclic aromatic alkaloids, cystodytins D [SI, E [3b], F 14a1, G [4b], H [Sal, and 1 [Sb] have been isolated from the Okinawan marine tunicate Cystadytes dellecbiajei and their structures elucidated on the basis of spectroscopic data.Recently a series of polycyclic aromatic alkaloids with structural similarities have been isolated from sponges, tunicates, or sea anemones and shown to exhibit interesting biological activities ( 1). We previously reported the isolation and structural elucidation of the tetracyclic aromatic alkaloids cystodytins A [la}, B [Ib}, and C 121, from the Okinawan marine tunicate Cystodytes dellecbiajei (della Valle 1877) (family Didemnidae), and we showed that these alkaloids possess potent cytotoxic activity as well as powerful Ca'+-releasing activity in the sarcoplasmic reticulum (2). In our continuing studies on bioactive substances from Okinawan marine organisms (3-7), we have further investigated extracts of the tunicate C. dellecbiajei to obtain six new strongly cytotoxic tetracyclic aromatic alkaloids, named cystodytins D [3a], E [3b], F [4a}, G [4b}, H [5a), and I [5b}, together with cystodytins A [la} and B {lb}. In this paper we describe their isolation and structure elucidation.