Chemical evaluation of the saline fermentation broth of several strains of the obligate marine actinomycete Salinispora arenicola has led to the identification of three new macrolide polyketides designated arenicolides A-C (1-3). The planar structures, elucidated via spectroscopic and chemical methods, consist of 26-membered polyunsaturated macrolactones containing repeating vicinal hydroxyl methoxyl moieties. The relative and absolute stereochemistries of 1-3 were assigned by a combination of J-based configurational analyses and chemical derivatization.