The sponge Darwinella rosacea was found to harbor a host of marine microorganisms. One of these bacteria was identified as a marine Alteromonas species. The organic extract of this bacterium was antimicrobial, and the activity was traced to two novel pseudomonic acid derivatives 1 and 7. The structures of these compounds were elucidated by interpretation of the spectral data and extensive use of 2-dimensional NMR techniques and NOE studies on the parent compounds and the acetate derivatives 2 and 8.