The structures of janthitrems B and C, the two most abundant tremorgenic mycotoxins produced by certain isolates of Penicillium janthinellum from New Zealand and Australia, have been elucidated by NMR spectroscopy. The compounds differ only by the oxygen atom in janthitrem B at C-22 but closely resemble janthitrems E-F except for dehydration in the terminal isoprene of the diterpenoid moiety. The diterpenoid moiety of janthitrems B and C is identical to that of penitrem D and generally similar to that of all penitrems.