Six new bicyclic peptides, celogentins D–H (1–5) and J (6) have been isolated from the seeds of Celosia argentea, and the structures including its absolute stereochemistry were determined by using extensive NMR methods and chemical means. Celogentins E–H (2–5) and J (6) showed potent inhibition of tubulin polymerization, while the inhibitory activity of celogentin D (1) was modest. Structure– activity relationship study indicates that ring size of the bicyclic ring system including unusual bs -Leu, Trp, and His residues would be important for their biological activity.