Three new protease inhibitors, micropeptins SF909 (1) and SF995 (2) and microcin SF608 (3), were isolated from the hydrophilic extract ofa microcystis sp. waterbloom. The planar structure of compounds !-3 was determined by homonuclear and inverse-heteronuelear 2D-NMR techniques as well as high-resolution mass spectrometry. The absolute configuration of the asymmetric centers was studied using Marfey's method for HPLC. Micropeptin SF909 (I) inhibited chymotrypsin with IC50 of 4.0 I.tg/mL while micropeptin SF995 (2) and microcin SF608 (3) inhibited trypsin with IC~0's of 0.2 and 0.5 I.tg/mL, respectively. © 1999 Elsevier Science Ltd. All rights reserved.