Seven new didemnins—didemnins M (1), N (2), X (3), and Y (4), nordidemnin N (5), epididemnin A1 (6), and acyclodidemnin A (7)—were isolated from an extract of the Caribbean tunicate Trididemnum solidum. The structures of these compounds were assigned, based on FABMS, high-field NMR data, and chemical degradation studies. Biological activities of these peptides are also described.