An endophytic fungus identified as Penicillium sp. was isolated from the brown alga Fucus spiralis collected from the Shetland Islands, United Kingdom. Bioassay-guided fractionation of an extract of the fungus led to the isolation of cladosporin, epiepoformin, phyllostine, and patulin, all of which showed antimicrobial activity against either Staphylococcus aureus or Pseudomonas aeruginosa. Cladosporin has not previously been identified from a fungus of the genus Penicillium, and, despite being biosynthetically related, epiepoformin, phyllostine and patulin have not been previously reported from one source.