Eupholathone, a minor diterpenoid with an unusual tetracyclic skeleton, was obtained from the seeds of Euphorbia lathyris, along with two known lathyrane diterpenoids, euphorbia factors L₂ and L₃. The structure of eupholathone was elucidated by detailed interpretation of its spectroscopic data, especially two-dimensional nuclear magnetic resonance spectroscopy and high-resolution electrospray ionization mass spectrometry.