Myxobacteria are gliding bacteria that belong to the delta-Proteobacteria and are known for their unique biosynthetic capabilities. Among myxobacteria, Nannocystis spp. are most closely related to marine myxobacteria and their secondary metabolism has hardly been investigated. Phenylnannolones A (1), B (2) and C (3) were obtained from a culture of Nannocystis exedens that was isolated from the intertidal region of Crete. Compound 1 had inhibitory activity toward the ABCB1 gene product P-glycoprotein and reversed daunorubicin resistance in cultured cancer cells. Phenylnannolone A has an unusual structural architecture; it is composed of an ethyl-substituted polyene chain linked to a pyrone moiety on one side and to a phenyl ring on the other. The investigation of the biosynthesis with labelled precursors revealed acetate, butyrate and phenylalanine as building blocks for 1. The labelling pattern suggested novel biochemical reactions for the biosynthesis of the starter unit.