The genetically encoded, small-molecule chemical diversity of filamentous fungi is still largely unexplored and represents an attractive source for the discovery of new compounds. Here we report the production of new chlorinated bianthrones from coculture of two different developmental stages, or morphs, of a marine alga-derived Aspergillus alliaceus (teleomorph: Petromyces alliaceus) strain. The vegetative stage (asexual morph) can be separated from the morph that switched to sexual development (sclerotial morph); both produce distinct secondary metabolite patterns. Ochratoxin (1) was mainly found in the monoculture of the sclerotial morph, while the anthraquinone pigment nalgiovensin (2) was produced by the asexual morph. Surprisingly, combining cultures from both developmental stages in a coculture experiment changed the metabolite profile drastically. The chlorinated congener nalgiolaxin (3) was abundant, and newly produced bianthrones were found. Allianthrone A (4) and its two diastereomers [allianthrones B (5) and C (6)] were isolated, and the new structures were determined by extensive NMR spectroscopic analysis, supported by optical properties and X-ray crystallography. All metabolites were tested in antibiotic and cytotoxicity assays, and allianthrone A (4) showed weak cytotoxic activity against the HCT-116 colon cancer and SK-Mel-5 melanoma cell lines.