The present invention relates to a polymyxin derivative of formula (I) and pharmaceutically acceptable salts thereof. The invention further relates to their use in the treatment of a bacterial infection, in particular infections caused by Gram-negative bacteria.本發明係有關式(I)之多黏桿菌素衍生物及其醫藥上可接受之鹽類。本發明進一步有關其等在治療一細菌感染,特別是革蘭氏陰性菌引起之感染上的用途。