Two abietane diterpenes were isolated from cyanobacteria Microcoleous lacustris, 20-nor-3alpha-acetoxyabieta-5,7,9,11,13-pentaene and 20-nor-3alpha-acetoxy-12-hydroxy-abieta-5,7,9,11,13-pentaene. These compounds were assayed against Staphylococcus aureus, Staphylococcus epidermidis, Salmonella typhi, Vibrio cholerae, Bacillus subtilis, Bacillus cereus, Escherichia coli, and Klebsiella pneumoniae. Both compounds showed activity against S. aureus, S. epidermidis, S. typhi, and V. cholerae, but not against the other bacteria.