Marine ascidians have been shown to be a very rich source of unique and biologically active secondary metabolites that have attracted the interest of both chemists and pharmacologists. A major group of those metabolites are nitrogen-containing compounds, particularly aromatic heterocycles. As part of our ongoing investigation on bioactive compounds from marine organisms, we describe in this paper the isolation of simple peptides 1-5 from the polar cytotoxic extract (100% inhibition of KB cells at 10 µg/mL and 80% inhibition of P-388 cells at 10 µg/mL) of the ascidian Leptoclinides dubius (Sluiter, 1909; order Enterogona, family Didemnidae), collected in New Caledonia. From the cytotoxic extracts of L. dubius, N-(p-hydroxybenzoyl)-L-arginine (1), N-(1H-indolyl-3-carbonyl)-D-arginine (2), and N-(6-bromo-1H-indolyl-3-carbonyl)-R (where R is L-Arg (4), L-His (5), and the very rare amino acid L-enduracididine (3)) have been isolated and identified by spectroscopic data, hydrolysis, and comparison with authentic samples. This is the first time the very rare amino acid enduracididine has been found in a marine organism, albeit in very small amounts. In summary, L. dubius is the source, as other ascidians, of amino acid-derived metabolites. In this particular case, they are modified dipeptides where one of the amino acid constituents has lost two carbon atoms and the amino group.