A secondary metabolite different from PR-imine and PR-amide was produced in the liquid (YESC) and solid (buckwheat) culture medium of Penicillium roqueforti. We isolated and purified the compound in pure and colorless crystalline form. On the basis of elemental analysis, mass, H-1 and C-13 nuclear magnetic resonance, infrared, and UV spectroscopy, the compound was identified as PR-acid (C17H20O7) The structures of PR-acid and PR toxin (C17H20O6) are closely related. Moreover, we discovered that PR-acid disappeared concurrently with the PR toxin in the culture medium, Thus, we postulate that PR toxin is degraded to PR-acid in the culture of P. roqueforti.