The fungus Penicillium paxilli Bainier produces a metabolite paxilline (C27H33NO4) capable of inducing severe, sustained tremors in mice with an ED50 of 25 mg/kg and a relatively low toxicity (LD50 of 150 mg/kg in mice). Isolation procedures, IR, and UV data were reported in a preliminary paper. Here, the structure of paxilline (1) is reported. Paxilline crystallizes as large clear cubes from acetone-heptane mixtures, belonging to the orthorhombic space group P212121 (a=31.009(3), b=11.522(1), c=7.707(1) Å). X-ray diffraction data (2186 reflections measured, 1840 observed after Lorentz, background, and polarization corrections) were solved via a multiple solution tangent formula approach and refined (conventional discrepancy index 0.040) with solvent acetone and all hydrogens included. The structure features a non-linear array of six rings: an indole moiety fused to a cyclopentane ring (trans-fused to a cyclohexane ring forming a linear tetracyclic arrangement), an additional trans equatorial fused cyclohexane ring (chair conformation) causing a molecular bend, and a γ-pyrone ring. Hydrogen bonds include a linear one from indole NH to acetone oxygen (2.88 Å), an intramolecular OH bond (2.71 Å), and an intermolecular OH bond (2.81 Å). The 100 MHz PMR spectrum (acetone-d6, TMS) was analyzed with assignments, and the CD spectrum showed positive Cotton effects at 335 nm (+5.0×10³) and 300 nm (+1.09×10⁴), with a potential negative effect at shorter wavelengths. Paxilline is closely related to paspaline and paspalicine (from Claviceps paspali), and biosynthesis studies are in progress.